WiFi works fine, 3G - not so much.
Just tested it over WiFi, initiated a video call from my phone to my desktop computer and it worked great.
The first time I tried to call from my desktop to my phone, the ringtone played on my phone, I clicked ANSWER, and my phone went back to the desktop with no sign that there was a video call. I ended the call from the computer side and tried again, and it worked fine that time.
No sign of "network connection" errors.
When I tried it on 3G, I initiated the call from the computer. When I went to try and answer it on the phone, my 3G data connection completely went away and I'm sitting on three bars of voice (I can send SMS) but no data, even after toggling Airplane Mode. I turned the Internet connection specifically off and on and I'm back up on 3G now.
Initiating the video chat from the phone had the exact same result. Except after I waited a couple of minutes, the 3G recovered on its own this time unlike the first time.
So, yeah, I'd say something's messed up trying to video chat with Google Talk over 3G, at least. BADLY messed up.