hi, was searching the forum to see if its possible to get google talk on the iphone. I found some posts from late 2006 that said it was not possible...
You can use webapps like Meebo and BeeJive to access GTalk on Safari. There are limitations with these, as Meebo doesn't run in the background.
If you're Jailbroken you can use FRING, which works really well (runs in the background with notification) - except it has been known to corrupt the SMS Database.