Just last week Google released Google Talk for the iPhone, this design is not only for the iPhone but for the iPod Touch as well. What this means is that you can send your friends Gmail from your iPhone or iPod Touch and actively use the Google Talk application while you’re on the move.
The way to use this is as follows: point your Safari browser to www.google.com/talk , (it would be a good idea to bookmark this page in your Safari browser) and follow the instructions.
There has to be differences from using Google Talk on your computer and from using it on the iPhone. "For
instance, in order to receive instant messages with Google Talk on your
iPhone, the application needs to be open in your Safari browser. When
you navigate away to another browser window or application, your status
will be changed to "unavailable" and your Google Talk session will be
restarted when you return." said The Official Google Mobile Blog.