Microsoft Codename Istanbul does just that... and like a knight you have to be annointed by Sir Bill's chosen clergy to get in... (read special permission from your account manager). I doubt we'll get a chance to use it at my customers' site because of their absolutely irrational FEAR of instant messaging.
My boss (my customers' CIO) recently told me that the reason IM is verboten is twofold: 1) Remote IM clients make a direct connection to our local IM clients, and 2) It's not encrypted. Little did I know, IM is apparently a type of P2P. As far as encryption - who cares unless that data is classified?
I wonder if Istanbul supports encryption - might be worth asking. If so, I'll ask Microsoft nicely if they'll consider us worthy enough to test Istanbul.
Update: I didn't believe IM was a type of P2P app. Check corrected me. Doh!