This took me way too long to figure out. I had a series of div tags that had mousover and mouseout events attached to them using prototype. I couldn’t get these events to fire in IE6 no matter what I tried! Events worked on links, but not any other element.

First thing to check: Are you using the png filter trick?
filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(enabled=true, sizingMethod=crop src='/images/white_bg.png');

If so, make sure your mouseover elements have “position:relative” or else the event won’t fire on any element inside another element with the png filter.

This post along got me on the right track.