FCC Chairman Kevin Martin said that the commission is against allowing mobile phones on planes because the networks on the ground probably can't handle the calls. The FCC has reportedly received thousands of emails asking for it to make this ruling because allowing flyers to use mobile phones would be annoying, but Martin's reasoning comes from the wireless industry. Wireless networks are designed to allow phones to connect to one cell tower at a time, but users on airplanes will connect to multiple towers simultaneous, possibly causing too much traffic for the networks.
For more on Martin's decision:
- see this New York Times article