As discussed on a number of threads in different FT forums, the regulation is not an FAA safety regulation. It is a Federal Communications Commission regulation meant to protect the national cell system from disruption caused by phones rapidly switching towers. The FAA specifically states that it does not regulate the use of cellular phones. Though, it could be argued that airlines are required by the FAA to enforce their rules, and most airlines have a rule that you have to turn off cellular functions from when the door is shut until after the airplane has landed.
Here is the link to the FCC regulation:
http://www.ecfr.gov/cgi-bin/text-idx....1.1.2.8.27.12
There is a note that says that the FAA regulates the use of cell phones on the ground, but, in fact, the FAA does not actually have any regulations.