Google Talk Optimized for iPhone
Although it seems pretty clear that there's going to be a native chat client for the iPhone 2.0 that will support Google Talk (with background alerts to boot!), there may be those that have a religious injunction against installing applications. There may also be those who don't intend to upgrade because they don't want to mess around with losing either their unlocked or jailbroken status. Whatever your reason for not using a native app, Google's got ya covered with an all new interface for Google Talk, fully optimized for Mobile Safari.
It's cool and all, but the moment you navigate away (or close) from Safari your status is set to unavailable. On the bright side, though, if you're a Google-holic it integrates nicely with your contacts and otherwise works exactly how you'd expect a Google Talk Client (inside a web browser) to work.
Just point your iPhone to http://www.google.com/talk to give it a shot.