The Google Talk blog has news of a Web-based Google Talk client for the iPhone:

Attention iPhone owners! You can now chat with all your Google Talk buddies while on the go. Our new version of Google Talk is designed specifically for the iPhone and runs in the iPhone's browser, so you don't need to download or install anything. Just visit www.google.com/talk on your iPhone, sign in, and start chatting. And because it is built for the browser, it will work on today's iPhones as well as on tomorrow's 3G iPhones.

Here' a shot from Google Blogoscoped:

I'd be all over the PC-based version of Google Talk if it could just interop with Windows Live Messenger, where all my contacts are. I just love the Spartan nature of the app. (And seriously, that iPhone client is prettier than the desktop version, go figure.)