Google Talk's video chat over 3G blocked by T-Mobile
When a T-Mobile Nexus S user tries to use the Google Talk video chat, he/she gets a message that says, "You must be connected to a WiFi network to start a video chat."
As the latest batch of Android phones gets launched, most are coming equipped with front-facing cameras. Once the new devices are launched with Android 2.3.4 and the other models receive upgrades to that build, the number of phones able to use Google Talk's video chat will grow in number tremendously.
source: AndroidCentralForums via Google