If you are an iPhone user and a fan of Google, you may want to check out a newly released version of Google Talk that is customized for the iPhone. The new Google Talk application is built for the iPhone as well as the iPod Touch. You can sign in from your iPhone it just by visiting the Google Talk site.
It doesn't feature quite the same functionality that the standalone clients do or that logging into Gmail accomplishes, as they admit, but should "remain true" to the Google Talk experience, as they say. It's a browser-based program so no installation is required, though I can easily see the desire for a standalone app on the iPhone as well.