If you are currently running an M2 version (zcs-3.0.0_M2_740 or 746) and want to upgrade to 3.1.1, you must perform the workaround in the following thread:
http://www.zimbra.com/forums/showthread.php?t=2581

If you don't perform the workaround before attempting the upgrade, then the installation will abort half-way and you will have to revert to your M2 backup.

According to Bugzilla, this bug is targetted for Edison (3.2):
http://bugzilla.zimbra.com/show_bug.cgi?id=7385