Groove IP is a battery hog and uses PCMU, which is not suitable for 3g connection. Can we stick to discussion on receiving GV calls in the native Google Talk app in android?
The native Google Talk app on Android is not a VoIP app. It is merely an instant messaging app. (It's also used behind the scenes for several other Google app services.)
To do what you want, several things would have to happen:
1. Google would need to rewrite the Google Talk app so that it adds VoIP functionality.
2. Google would need to update the Google Voice app on Android so that it could interface with the Google Talk app.
3. Google would have to modify the Google Voice service so that it could forward to the Google Talk service on an Android device... and differentiate between forwarding to a cellular number versus forwarding to a VoIP client.
4. Making those changes would undoubtedly break any 3rd-party apps that interface with Google Talk or Google Voice. So new APIs would need to be published for devs to be able to update their apps accordingly.
Not that it wouldn't be a nice feature to have as part of the OS, but I wouldn't expect it to happen overnight. It would most certainly alienate the carriers. I'm sure they would do everything they could to disable it or block it.
Personally, I don't have a need for VoIP via my cell. I have unlimited talk minutes anyway. I suppose there is a use case if you're without a cell signal in a basement or building and want to use WiFi to make/receive calls.
Anyway, my point is it wouldn't be a trivial matter to implement.