Some users have reported an issue in which their Macs no longer recognize mouse clicks from some USB mice after the Mac OS X 10.6.3. The problem may occur at random or after the Mac is woken from sleep.
Possible fixes
Clear kernel caches. Some users have also reported at least temporary respite from this issue via clearing of various caches (including kernel extension caches) with a tool such as AppleJack. To follow this routine, download AppleJack and install it, then restart your Mac in single-user mode by holding down the Command and S keys at startup. At the prompt, type “applejack auto restart” (with no quotation marks) and press return [note that while AppleJack 1.5 is technically incompatible with Snow Leopard, only the “repair permissions” function fails, which is not necessary for this process. If you’d like, you can use an alternative cache clearing tool such as YASU.]
Reset PRAM/NVRAM. This fix has worked for previous instances of this issue. See this Apple Knowledge Base article for instructions for performing this procedure.
Remove third-party startup items. Third-party startup items that are incompatible with Mac OS X 10.6.3 may cause this issue. Look in the folder /Library/StartupItems/ for any third-party startup items and temporarily remove them then restart.’
Downgrade to Mac OS X 10.6.2. As a last resort, you can downgrade to an earlier version of Snow Leopard using the “Archive and Install” feature from your Mac OS X installation disc as described here. Note that Apple made changes to “improve the reliability of certain third-party USB devices” under Mac OS X 10.6.3, which may be the cause of the issue.