According to The Official Google Mobile Blog, simply point your iPhone's browser to http://www.google.com/talk. The software is entirely browser based, so there's no need to download or install anything. (Google didn't say if it is working on a downloadable version that will work with the iPhone 2.0 firmware, due next week.)
Google pointed out:
I gave it a whirl first thing this morning. It works pretty well. The need to have an active browser window open is a bit of a hamstring, but it's better than not being able to chat at all.
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.
I am still looking forward to standalone IM programs for the iPhone to become available from the iPhone Apps Store next week. In the meantime, this gets the job done.