The lack of a true instant messaging client for the iPhone has been an issue for some. Meebo.com is a popular web-based client which offers support for AIM, Yahoo, Google Talk, and MSN -- all in one interface.
Unfortunately, the iPhone was unable to use the Meebo interface due to the lack of a "Send" button. iPhone users were unable to otherwise submit their instant messages.
It looks like the Meebo team has responded and made the appropriate changes, so Meebo now works from the iPhone. The "Go" button appears on the iPhone keyboard and submits your messages.