i LITERALLY noticed this yesterday to GREAT agitation whilst playing team fortress 2.
i have also recently upgraded to win7 and neither the logitech set-point software (for my g5 gaming mouse) nor the system-settings changed anything. i went away and replaced my steelseries steelpad with another spare one i had lying around, thinking the traction/quality on the surface had finally eroded away. i changed my in-game sensitivity, messed around with the DPI of the mouse and even upped the refresh rate to the maximum amount possible on the USB2.0 interface. mouse accel and all forms of pointing-aids have been disabled. the mouse feels slower or like it's 'missing' traction whenever i move horizontally, from left-to-right. up and down axis feels fine.
it's an eternal fucking pain in team fortress 2 and yet another reason why i am lamenting ever choosing to 'upgrade' to this piece of shit. so far windows 7 has fucked with my sound (messing up my EQ, my settings, and now half of my in-game noises just don't play altogether), my mouse settings (for what reason, i do not know) and my file organisation. this OS sucks dick.
best of luck finding a solution-- im definitely interested in finding one, too!
libertarian benefit collector - anti-academic super-intellectual.
http://mixlr.com/the-little-phrase/