Well the next iPhone software will most likely be 2.0 in June, with that comes the release of 3rd party apps, so messenger applications etc will probably be available.
@ cwilbur – Well AIM have made one, it won’t run in the background but it’s an IM client, granted not in the sense that most others work (run in background, alert of new IM’s etc) but I’m sure there will be IM clients for it.