updated 09:50 am EDT, Thu March 15, 2012
Carriers likely to blame
Apple is continuing to limit FaceTime calls to Wi-Fi connections, even on 4G-equipped iPads, The Verge notes. Attempting a FaceTime call on 4G brings up the same error message Apple has used with 3G iPhones and iPads. "Connect to a Wi-Fi network to use FaceTime," a pop-up warns.
The potential quality of calls has sometimes been brought up as a reason for limiting FaceTime to Wi-Fi, but on a 4G connection the issue should be moot. LTE networks are capable of operating at a higher bandwidth than landline connections, often in excess of 20Mbps. Pressure from carriers may therefore be responsible for keeping the cellular restriction intact.
In 2010, then-Apple CEO Steve Jobs claimed that the company needed to "work a little bit with the cellular providers to get ready for the future, so we're Wi-Fi only in 2010." Carriers may be worried about the congestion video calls can create, and possibly diminished need for voice plans. At the same time though, apps like Skype can already operate over 3G, 4G, or Wi-Fi.