I just caught this thread, I have been running 2.1 as soon as it came out, against our 5.0.8 server.
I just had my cohort send me an invite and they seem to be working--I hit "Tentative" and waltzed away from it, came back and opened it and hit "Accept". It DID leave my Invites drawer after accepting, but if I go right to the event in the calendar I can still, for example, decline.
In fact as I write this I watched it push the changes to my Zimbra calendar--the event just ghosted.
Chris @ DPM
Version 6.0.16_GA_2998.NETWORK Apr 4, 2012 MACOSXx86
Xserve 2x Dual Core Xeons 10GB with Xserve RAID