My Logitech mouse stopped being recognized in Snow Leopard. Logitech, in their infinite wisdom, has their Logitech Control Center 3.0 installer recognize and defeat any attempt at installing on any system other than 10.5.
The simple solution is to view the contents of the installer .pkg file (Control-click on it an select Show Package Contents), then navigate to Resources » Logitech Control Center.mpkg and double click. This launches the installer with no system version test, and re-installs the components in the correct locations. Restart and voilá -- mouse recognized!
[robg adds: I don't have a Logitech mouse to test this one with.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090902043559141