The following entry was posted on the Google Mobile Blog.
We've just 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 while you're on the move, too! In your iPhone browser, just go to www.google.com/talk, sign in and start chatting. That's it. Google Talk runs entirely in the browser so there's no need to download or install anything.
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.
Other than that, we've tried to keep the design as faithful to the desktop experience as possible, so it should be familiar to you. You can select from a quicklist of the people you contact most, search your contacts, and manage multiple conversations. You'll never have to miss out on the latest gossip or updates from friends just because you're away from a computer.