Concurrent user login
Would it be possible to add a feature to limit the number of concurrent logins by a user, it would seem a bit silly to leave a current session open and allow the user to login at a different machine. You could also close other sessions (possibly) if the user logs-in elsewhere.
If your worried about seasons on the server it's not a problem. The session and caches on the server would not be duplicated but shared. The only thing you'd lose perf wise are the extra server hit to keep multiple clients in sync. To be honest this is a feature of Zimbra to be able to keep various clients all in sync at the same time. I personally love the fact I can use IMAP on my PC or phone sync Outlook and my PDA, go into the data center and pull up my mail on the webclient to check something then go back to my desk and still have the other clients active. If the server kicked me out each time I changed clients I think this would cost me productivity. Do you have a different set of use cases in mind?
The only problem I have with that is security, what happens if a user has confidential email and a 'passer by' has access to the mailbox while I'm somewhere else? It's not a problem for me but I do know sites where that might be an issue. I'm only suggesting that to have an admin (and/or user) defined option limiting the number of concurrent logins might be useful for some sites.