First of all, good luck for your project!!
I don't understand why users should use your server either. Just let them connect directly... That way you won't have to worry about agreements or permissions either. That should also get rid of several privacy questions ("why should this application upload my password?")
I'm not sure about official permissions. I've just noticed that eBuddy randomly picks one server out of a pool, so maybe you shouldn't have several connections from the same server after all. In any case, I don't see a good reason to not connect directly.
If you don't mind a personal request... I'd love a chat client for my Symbian phone, where I can open the application, type my password (NOT save it! Not even an application password to access my MSN password that's stored elsewhere!), login invisible, start chatting as invisible, and have my PSM intact back when I login with aMSN!