Since the icon is right there on your iPhone home and you signed up for all those extra or unlimited SMS messages, maybe you didn’t consider using IM.
Or maybe you did, and I’m just slower than you ;) Well, I had mine for 20 minutes and immediately decided that free data is better than paid text.
Maybe the easiest way to IM on your iPhone is to point Safari to your meebo account (a web-based chat aggregator).
However, there are other apps out there (including even BitchX, my favorite CLI irc client).
Right now I’m trying out JiveTalk…read about it.
Or start using by pointing your iPhone Safari.
Others to consider:
Apollo (no gtalk/jabber yet)