updated 03:40 pm EDT, Wed June 8, 2011
Would likely be carrier-toggled
The iOS 5 version of FaceTime will finally allow video calls over 3G, rumors suggest. Sources indicate that the functionality is simply turned off in developer builds, and that an error message urges people to "Turn on cellular data or use Wi-Fi to use FaceTime." It's believed that support will ultimately be dependent on carriers, who can already block things like tethering.
Apple may be waiting on an improved iPhone before turning on FaceTime. Like iOS 5, the next iPhone is expected to ship in the fall. While it's unlikely to support LTE, it may support HSPA+ speeds up to 21Mbps, which would make 3G-level video calling more practical. Streaming video tends to degrade over regular HSPA, which has real-world bandwidth closer to between 1 and 3Mbps.