They did a segment on MythBusters about this.
From here:
Myth: Cellphones don’t interfere with plane instrumentation—it’s a conspiracy to get passengers to pay the higher priced in-flight phones.
Result: Using a faraday cage to block outside interference, the MythBusters set up a few common instruments from jetliners inside while using a cellphone. At 800–900MHz (a common cellphone bandwidth frequencies), there’s definite interference on some of the aircraft instrumentation. Because of federal law, they can’t test in the air, though they’re able to do so on a grounded private plane., and the tests there show no interference at all. The MythBusters call the conspiracy busted because they believe the airlines are simply not taking any chances with cellphones, since new ones with new frequencies are coming out every day.