There's an app for that if you want your chats going to that called Fringe. Though it might work if you add your account as an Exchange account.
VOIP app for iPhone called Fring:
http://www.fring.com/googletalk/
Never mind, I don't know if it does Voice.
If you wanted Google Voice, and you had account from Grand Central days, you can use any VOIP app that works with Gizmo. I don't know if that ties to Google Talk's "voice" either.