One thing about aero though - even with the taskbar always on top and programs do correctly stay above it, wallpapers go all the way to the bottom of the screen including under the taskbar:
I don't recall whether this was the same back in XP/pre-aero, though I have a feeling it was. It's just more noticeable now since the aero taskbar is semitransparent. I have wallpaper slideshow running at the fastest setting (10 seconds) so I can see the changing wallpaper through the taskbar.