This issue exists across all browsers I've seen to date. onmouseout, onmouseover, and even the CSS property :hover are triggered only when the mouse is moved over or off an object. But if an object slides under or away from a static mouse on its own, either by page scrolling or animating the object's location, the events aren't triggered. This may seem insignificant, but in a more elaborate scenario such as the sliding icons at the top of my albums: http://homepage.mac.com/samrod/photos/sailingWariane , the icons don't respond as expected while in motion.
*** This bug has been marked as a duplicate of 4117 ***