Cell phones and avionics was a Mythbuster's topic one time. I can't remember the details, but the testing initially showed cell phone signals possibly affecting a navigation instrument (a VOR thingamajigee?), but I can't recall if they had to crank up the signal strength beyond normal levels to achieve that. However, with further testing, they weren't able to affect any avionics.

Actually, since there's talk of allowing cell phone use on commercial aircraft, I guess the telecom companies have figured out how to deal with multiple hits to their system from the same phone.