Google has just announced Google Talk for the iPhone (and iPod touch). You won't have to wait for the App Store to use it, since this is a web app designed for the iPhone's browser (remember when Apple wanted us to believe that web apps were just as good as fully native apps? My how times have changed).
All you have to do is point your iPhone to www.google.com/talk and soon you'll be chatting away with your Gmail contacts. Since this is a web app, Google Talk for the iPhone only works when it is the active window in Mobile Safari. Close the web broswer, or switch to a different window and you'll show up as unavailable in Google Talk.