ok, no lleva php, es puro javascript, es algo extensa la función.

el javascript:
[code type=javascript]
/*!
*
* Portamento v1.1.1 – 2011-09-02
* http://simianstudios.com/portamento
*
* Copyright 2011 Kris Noble except where noted.
*
* Dual-licensed under the GPLv3 and Apache 2.0 licenses:
* http://www.gnu.org/licenses/gpl-3.0.html
* http://www.apache.org/licenses/LICENSE-2.0
*
*/
/**
*
* Creates a sliding panel that respects the boundaries of
* a given wrapper, and also has sensible behaviour if the
* viewport is too small to display the whole panel.
*
* Full documentation at http://simianstudios.com/portamento
*
* —-
*
* Uses the viewportOffset plugin by Ben Alman aka Cowboy:
* http://benalman.com/projects/j-misc-plugins/#viewportoffset
*
* Uses a portion of CFT by Juriy Zaytsev aka Kangax:
* http://kangax.github.com/cft/#IS_POSITION_FIXED_SUPPORTED
*
* Uses code by Matthew Eernisse:
* http://www.fleegix.org/articles/2006-05-30-getting-the-scrollbar-width-in-pixels
*
* Builds on work by Remy Sharp:
* http://jfordesigners.com/fixed-floating-elements/
*
*/
var j = jQuery.noConflict();
(function(){

j.fn.portamento = function(options) {

// we’ll use the window and document objects a lot, so
// saving them as variables now saves a lot of function calls
var thisWindow = j(window);
var thisDocument = j(document);

/**
* NOTE by Kris – included here so as to avoid namespace clashes.
*
* j viewportOffset – v0.3 – 2/3/2010
* http://benalman.com/projects/j-misc-plugins/
*
* Copyright (c) 2010 “Cowboy” Ben Alman
* Dual licensed under the MIT and GPL licenses.
* http://benalman.com/about/license/
*/
j.fn.viewportOffset = function() {
var win = j(window);
var offset = j(this).offset();

return {
left: offset.left – win.scrollLeft(),
top: offset.top – win.scrollTop()
};
};

/**
*
* A test to see if position:fixed is supported.
* Taken from CFT by Kangax – http://kangax.github.com/cft/#IS_POSITION_FIXED_SUPPORTED
* Included here so as to avoid namespace clashes.
*
*/
function positionFixedSupported () {
var container = document.body;
if (document.createElement && container && container.appendChild && container.removeChild) {
var el = document.createElement(“div”);
if (!el.getBoundingClientRect) {
return null;
}
el.innerHTML = “x”;
el.style.cssText = “position:fixed;top:100px;”;
container.appendChild(el);
var originalHeight = container.style.height, originalScrollTop = container.scrollTop;
container.style.height = “3000px”;
container.scrollTop = 500;
var elementTop = el.getBoundingClientRect().top;
container.style.height = originalHeight;
var isSupported = elementTop === 100;
container.removeChild(el);
container.scrollTop = originalScrollTop;
return isSupported;
}
return null;
}

/**
*
* Get the scrollbar width by Matthew Eernisse.
* http://www.fleegix.org/articles/2006-05-30-getting-the-scrollbar-width-in-pixels
* Included here so as to avoid namespace clashes.
*
*/
function getScrollerWidth() {
var scr = null;
var inn = null;
var wNoScroll = 0;
var wScroll = 0;

// Outer scrolling div
scr = document.createElement(‘div’);
scr.style.position = ‘absolute’;
scr.style.top = ‘-1000px’;
scr.style.left = ‘-1000px’;
scr.style.width = ‘100px’;
scr.style.height = ’50px’;
// Start with no scrollbar
scr.style.overflow = ‘hidden’;

// Inner content div
inn = document.createElement(‘div’);
inn.style.width = ‘100%’;
inn.style.height = ‘200px’;

// Put the inner div in the scrolling div
scr.appendChild(inn);
// Append the scrolling div to the doc
document.body.appendChild(scr);

// Width of the inner div sans scrollbar
wNoScroll = inn.offsetWidth;

// Add the scrollbar
scr.style.overflow = ‘auto’;
// Width of the inner div width scrollbar
wScroll = inn.offsetWidth;

// Remove the scrolling div from the doc
document.body.removeChild(document.body.lastChild);

// Pixel width of the scroller
return (wNoScroll – wScroll);
}

// —————————————————————————————————

// get the definitive options
var opts = j.extend({}, j.fn.portamento.defaults, options);

// setup the vars accordingly
var panel = this;
var wrapper = opts.wrapper;
var gap = opts.gap;
var disableWorkaround = opts.disableWorkaround;
var fullyCapableBrowser = positionFixedSupported();

if(panel.length != 1) {
// die gracefully if the user has tried to pass multiple elements
// (multiple element support is on the TODO list!) or no elements…
return this;
}

if(!fullyCapableBrowser && disableWorkaround) {
// just stop here, as the dev doesn’t want to use the workaround
return this;
}

// wrap the floating panel in a div, then set a sensible min-height and width
panel.wrap(‘

‘);
var float_container = j(‘#portamento_container’);
float_container.css({
‘min-height’: panel.outerHeight(),
‘width’: panel.outerWidth()
});

// calculate the upper scrolling boundary
var panelOffset = panel.offset().top;
var panelMargin = parseFloat(panel.css(‘marginTop’).replace(/auto/, 0));
var realPanelOffset = panelOffset – panelMargin;
var topScrollBoundary = realPanelOffset – gap;

// a couple of numbers to account for margins and padding on the relevant elements
var wrapperPaddingFix = parseFloat(wrapper.css(‘paddingTop’).replace(/auto/, 0));
var containerMarginFix = parseFloat(float_container.css(‘marginTop’).replace(/auto/, 0));

// do some work to fix IE misreporting the document width
var ieFix = 0;

var isMSIE = /*@cc_on!@*/0;

if (isMSIE) {
ieFix = getScrollerWidth() + 4;
}

// —————————————————————————————————

thisWindow.bind(“scroll.portamento”, function () {

if(thisWindow.height() > panel.outerHeight() && thisWindow.width() >= (thisDocument.width() – ieFix)) { // don’t scroll if the window isn’t big enough

var y = thisDocument.scrollTop(); // current scroll position of the document

if (y >= (topScrollBoundary)) { // if we’re at or past the upper scrolling boundary
if((panel.innerHeight() – wrapper.viewportOffset().top) – wrapperPaddingFix + gap >= wrapper.height()) { // if we’re at or past the bottom scrolling boundary
if(panel.hasClass(‘fixed’) || thisWindow.height() >= panel.outerHeight()) { // check that there’s work to do
panel.removeClass(‘fixed’);
panel.css(‘top’, (wrapper.height() – panel.innerHeight()) + ‘px’);
}
} else { // if we’re somewhere in the middle
panel.addClass(‘fixed’);

if(fullyCapableBrowser) { // supports position:fixed
panel.css(‘top’, gap + ‘px’); // to keep the gap
} else {
panel.clearQueue();
panel.css(‘position’, ‘absolute’).animate({top: (0 – float_container.viewportOffset().top + gap)});
}
}
} else {
// if we’re above the top scroll boundary
panel.removeClass(‘fixed’);
panel.css(‘top’, ‘0’); // remove any added gap
}
} else {
panel.removeClass(‘fixed’);
}
});

// —————————————————————————————————

thisWindow.bind(“resize.portamento”, function () {
// stop users getting undesirable behaviour if they resize the window too small
if(thisWindow.height() <= panel.outerHeight() || thisWindow.width() < thisDocument.width()) {
if(panel.hasClass('fixed')) {
panel.removeClass('fixed');
panel.css('top', '0');
}
} else {
thisWindow.trigger('scroll.portamento'); // trigger the scroll event to place the panel correctly
}
});

// —————————————————————————————————

thisWindow.bind("orientationchange.portamento", function () {
// if device orientation changes, trigger the resize event
thisWindow.trigger('resize.portamento');
});

// —————————————————————————————————

// trigger the scroll event immediately so that the panel is positioned correctly if the page loads anywhere other than the top.
thisWindow.trigger('scroll.portamento');

// return this to maintain chainability
return this;
};

// set some sensible defaults
j.fn.portamento.defaults = {
'wrapper' : j('body'), // the element that will act as the sliding panel's boundaries
'gap' : 10, // the gap (in pixels) left between the top of the viewport and the top of the panel
'disableWorkaround' : false // option to disable the workaround for not-quite capable browsers
};

})(j);
[/code]

El portamento trae un css para que se agregue al div, que es este:

[code type=css]
#wrapper {position:relative;}
#menu {position:absolute; right:0; top:0;}
#portamento_container {position:absolute; right:0; top:0;} /* take the absolute positioning of the menu */
#portamento_container #menu {}
#portamento_container #menu.fixed {position:fixed; right:auto; top:auto;} /* become fixed position, but reset the top and right values */
[/code]

Lo he modificado ligeramente para ajustarlo a mi web.

El resto lo hace la funcion. Entra a la web y Mira con el firebug el div en cuestión que es el menú: Mundo Crece, observarás que el comportamiento del css varía dependiendo de si el scroll hace que el div tope el top o no, y aquí: Mundo Crece fallo el comportamiento del div donde aparece el fallo es diferente, no entiendo nada, pero no se comporta igual y la razón la desconozco completamente :/

Eso ya lo sé, a excepción del valor fixed, el plugin al que me refiero hace un efecto el cual no se consigue con un simple valor css. Si te fijas atentamente el plugin hace que el div tome un comportamiento muy diferente, echale un vistazo, el inicio y el fin del desplazamiento lo puedo controlar a mi gusto, no como el valor fixed, pero muchas gracias por tu respuesta Federico!