Google has finally launched a mobile version of Google Talk, but it's optimized for iPhone's browser. You can access if you go to google.com/talk on an iPhone/iPod Touch or using this permalink: http://talkgadget.google.com/talkgadget/m (shorter URL: http://tinyurl.com/4vnfcd).
This Ajax version is based on Google Talk's Flash gadget, but it doesn't include tabs, group chat, options to add contacts and send email. What you can do is to update your status, search your contacts and chat with people that are online.
Besides this mobile interface, Google also has a mobile app for Blackberry. That means if you don't have a Blackberry, an iPhone or at least a WebKit mobile browser, you have to find a third-party service.
eBuddy is a service that offers both a basic mobile interface for Google Talk and a mobile application that lets you chat faster with your friends. eBuddy doesn't connect only to Google Talk, it's a multi-network IM client that works with MSN, Yahoo and AIM. Unfortunately, I couldn't find any information regarding the way eBuddy stores or uses your credentials and seeing that eBuddy sends the password in plain text is worrisome.
Another mobile application that lets you chat with your Google Talk contacts is Fring, but it only works on Symbian and Windows Mobile devices. As usually, make sure you trust the service before entering the username and password of a Google account.