this is deffently a bug
i have acer aspire 5100
with amd turion 64 bit
ati raadon Xpress 1100
the same thing happend to me
and its gets worst
when the task bar is up it leavs a little shadow bar binith the task bar
and it covering other open windows
big big big bug
Have you ruled out the possibility of a bad driver? As in have you installed the latest Windows 7 driver for your gpu? And if that didn't work or wasn't available for your specific model than have you tried the latest available Windows Vista driver? I only ask this because I know there were a couple others in a different thread with a very similar problem and it was just a matter of trying a different driver for them..
after moving the bar to the bottom and back to the top to get rid of the extra taskbar piece, maximizing a window still only maximizes to where that extra taskbar piece was ... until i change to large icons then back to small and put back on the bottom and then back on the top again.