The flight attendant just showed me the United policy manual which prohibits "two way devices" from communicating with the ground. However, the PLANE HAS WIFI. To combat this, not unlike China, United and other airlines have blocked Skype and other known video chat offenders. Apparently, they missed Apple iChat. Oops.Some have pointed out how terrorism strikes on two main fronts: the initial loss of life and property caused by the attack, and the ongoing fear and lifestyle changes it provokes. September 11 happened more than 8 years ago, yet it still significantly delays air transit at substantial taxpayer expense because of the security measures that have been put in place.
Allowing access to the Internet on planes but blocking certain activities is doomed to failure. There are innumerable ways that a could terrorist could send a message via WiFi to people on the ground besides video chat, and the authorities can't possibly account for all of them.
The Boing Boing article also details another reason to ban video chats: like cellphone conversations, they annoy nearby passengers.