Minou chat Chat is a web application for the iPhone and iTouch that allows you to chat in different discussion-rooms. Among its features are the fact that it is very light so it does not consume a lot of battery.
The app also contains a number of games in IRC style.
One of the nice other options you have, is the possibility to create your own chat rooms.

No registration required, just insert a nickname and a password.

Enter
http://mac-gratuit.fr/iphone/minou/
in your mobile safari

Enjoy