The Network Edition has IM Interop - at this time just YIM (Click the Y! logo on the bottom right of the IM tab) AIM & MSN are technically possible through protocol extensions but I haven't checked into that in a while.
However, if you're using the Open Source Edition you can still add any XMPP friends (I can add buddies from ZCS>gmail & gmail>ZCS with no problem/get invites at the other end) -
SRV records are good to have.