Last week, we took a look at Meebo, a universal chat client that you use through the Web. We looked at Meebo as an option to do IM on the iPhone. Earlier this week, an alpha version of Beejive's JiveTalk for iPhone was unveiled.
JiveTalk is compatible with AIM/iChat, GoogleTalk, ICQ, Jabber, MSN, and Yahoo. It's free, but since the software is at the alpha stage of development, it still has bugs, though we didn't encounter any. If you're looking to save your SMS count, you can use JiveTalk.

To try JiveTalk, fire up Safari on your iPhone and go to iphone.beejive.com.

We added an AIM account. AIM is compatible with iChat. If you want to add an account with a different service, then select it from the list.
Next: Chatting on JiveTalk.

After you log in, you see your buddy list. If you have multiple IM accounts (AIM and Yahoo, for example), your buddies are grouped together.

Typing and chatting (below) on JiveTalk looks just like SMS on the iPhone.

TwitterIM was the only "responsive" buddy we had at midnight last night.
JiveTalk seems to have the interface down, an advantage it has over Meebo. Eventually, we were able to chat with three different people without a hitch.
To try JiveTalk on your iPhone, launch Safari and go to iphone.beejive.com. Try it when your buddies are awake.
Links:
[1] http://www.maclife.com/article/how_to_instant_message_on_an_iphone
[2] http://www.beejive.com
[3] http://iphone.beejive.com
[4] http://www.maclife.com/article/5_more_iphone_tricks
[5] http://www.maclife.com/article/how_to_remove_the_iphone_sim_card
[6] http://www.maclife.com/article/goffice_word_processing_on_the_iphone