My current client is a media company who wanted branding done as part of a SharePoint 2010 Portal implementation project. One problem arose because their standard operating environment uses Internet Explorer 7. Our IE 7 specific issues arose because of IE7's position behaviour and it's interaction with a dynamic pop-down DIV that we were injecting with jQuery.
In IE7 (not in IE8+ or Firefox), our whole content div in SharePoint was getting pushed down by our hidden div used as part of the pop-down menu. Turns out our resolution to fix this IE 7 was to make our hidden div to use position:absolute rather than relative/static.
The critical difference between relative and absolute in particular is not how it changes the behaviour of the div - but rather how it affects the flow of OTHER elements on the page. This is particularly important when you are injecting elements into SharePoint with jQuery. The best explanation of CSS positioning (and how it affects other html elements) I've found is here:
RELATIVE: The content of relatively positioned elements can be moved and overlap other elements, but the reserved space for the element is still preserved in the normal flow.
Absolutely positioned elements are removed from the normal flow. The document and other elements behave like the absolutely positioned element does not exist (Yes! this is what we really want)
This still doesn't explain the rendering difference in IE7 vs IE8 vs Firefox apart from the fact that other browsers seem to be more forgiving when specifying your CSS positioning compared to IE7 (in Standards mode).