i had this problem and ati customer support e-mailed me...
"you must have the ati video card installed in order to remove the drivers"
in other words, if your card breaks or you upgrade, you cant use the uninstaller.
ati's drivers really suck imho if they force this.
if your card is not installed, try putting the ati card back and then uninstalling if you want to use the uninstaller.
good luck.