Google Talk isn't the only iPhone optimized IM client

I think Meebo was one of the first to launch an iPhone-optimized interface and it works very well. Simply go to Meebo.com on your iPhone or iPod Touch to get started. Meebo supports Google Talk, AIM, MSN, Yahoo, ICQ, and Jabber. You can select and sign into one client at a time with Meebo and each client is optimized for the iPhone browser.
If you want to chat with your buddies via AIM, MSN, Yahoo IM, MySpaceIM, ICQ, or Jabbber and sign in with multiple IM clients on a single interface then simply go to Giz5.com to get started. If you don't yet have a Gizmo account you will need to sign up for free. The interface is decent, but could still use some improvements (larger default fonts and layout) to work even better with the iPhone.
There are other mobile clients available too and I recommend you check out Robin Good's Mini-Guide to see all that are available with a comparison chart.