Google has released voice talk for mobile phones. For Android and Blackberry they have it as an application. For other mobiles such as iPhone, Nokia they have it as a web service.
More details are given in http://www.google.com/mobile/voice/
For mobile: http://m.google.com/voice
I have been looking for something similar to google talk for mobile for a long time. Now it’s the time google got the way around.