Wish I could be more specific, but we need more information.
You might need to check some log files...look at your /var/log/Xorg.0.log for warnings and errors.
'grep' your log directory looking for references to your mouse...
Code:
fgrep -i mouse /var/log/*
...or something like that.
Another potential slip might be some package upgrade affecting gtk+ (or bonobo?). Check your '/var/log/aptitude' log or your '/var/log/dpkg.log' for half configured packages in recent upgrades.
Post the things you find unexplained or suspicious.
