Fring
Who would have thunk it? Fring is the world’s first VoIP for the iPhone and not only that, it runs in the background! And yes, you can hook up MSN, ICQ, Skype and the likes to your account and perhaps permanently never leave* your chat client for good.
* That was a joke. Never have this running at all time, please have some conversation with real life people. The kind you see face to face every day…