I'm not sure if this is a new or existing/identified problem. I did some brief searching for it on the forums so I figure I'll post a new thread.
I recently upgraded to 5.0.18 NE - haven't seen any problems except this -- user accepts invitation via iCal but the "reply to" mail header is from the user's alias and not his mail email address.
The user has 5 aliases -- if I reconfigure the top top-most one as a Distribution List, then iCal will use the next alias. And so on down the line. In between each test, I blow away the iCal profile/account each time.
In the end, I just recreated his aliases as Distribution Lists. Anyone else have/seen this problem/quirk?