Google has launched Google Talk for mobile, a version, which’s optimized for iPhone's browser at the moment. 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.
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 multi-network IM client service that connect to Google Talk, MSN, Yahoo and AIM that lets you chat faster with your friends. 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.
Source:→ Google System