Google released in the US a new version of Google Talk designed specifically for the iPhone and iPod Touch browsers.

In addition to sending your friends Gmail messages from your iPhone, you can now chat with them too.
Google Talk for the iPhone is just a web service so no apps need to be installed.
There are some differences from using Google Talk on your computer. For instance, in order to receive instant messages with Google Talk on your iPhone, the application needs to be open in your Safari browser. When you navigate away to another browser window or application, your status will be changed to "unavailable" and your Google Talk session will be restarted when you return.

Point your iPhone Safari browser to www.google.com/talk to start chatting on your iPhone. Via the Google Mobile blog. More iPhone News.