Although this thread started with a discussion of Blackberry devices, it has quickly been pulled into all RF and electronic devices. There are two sets of rules concerning cell phone usage. The FCC is concerned that several dozen cell phones turned on while overflying an area can cause issues for the cell towers that are in range. Secondly the FAA is concerned about interferenc with internal aircraft systems. This is part conjecture on my part, a single cell phone or a few turned on are not really a problem, but what about 50 or 100 or more on large jets? I also think the "turn off all electronic devices" before takeoff and landing has more to do with keeping people focused on responding to an emergency if one should occur -- as opposed to any potential interference issues.