The power density from a 5MW transmitter at 1km and 100 10W cell phones at 10m is about the same. But you have to remember that the navigation antennas are outside the metal body of the plane, so the phones probably don't matter.
Sometimes you hear interference from GSM phones because the TDMA time divisions are in the AF range, but that's really a function of the power envelope rather than the power of the transmitter. I don't even really count this because I've never seen a GSM phone interfere with a non-part-15 device. Radios have to reject nearby signals; noise well outside their frequency range is even easier to reject. (I've never heard anything like GSM interference while operating my HF radio, either, and I have a GSM phone.)
I'm sure it happens (the post a bit back about the guy's laptop that was broadcasting mic input due to wacky wiring is a great example), but I'm really dubious that it's significant enough to actually interfere with comms from a system designed to throw radio signals 200 miles.