Google has officially announced that video chat is coming to Nexus S devices as part of the Android 2.3.4 update which they will release in an over-the-air update in the next few weeks. You will be able to make calls over a 3G or 4G data network or WiFi. This update will also launch on other Android 2.3+ devices later on, but Nexus S first as alway.

Source: Google Mobile Blog