My Blue MS Optical mouse was causing me problems (the middle click upset menu and dock sensitivity ... but that's another story!), so I installed the MS Intellipoint driver v5.0. This enabled me to map the mouse buttons to all kinds of things, even different things for different apps (much like USB overdrive, but free). This was great, except that the mouse acceleration speed was inconsistent -- fast user switching and going back to the Login page would alter the mouse speed. The MS driver preference pane let you use the OS X mouse acceleration, but seemed to keep overriding the OS X setting when changing apps (usually back to a slow crawl). The solution to stop this interference is as follows:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040210132825258