Actually, the chance of a cell phone at 900MHz or 1.8 or 1.9Ghz interfering with the radio communication of an airplane with proper shielding is as near zero as it gets. Shielding of both phones and aircraft can degrade over time, but there is little evidence to suggest that even this can cause real (compared to theoretical) interferance.
(And how many of you actually turn off your computer's wireless connection when you fly?).
http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft