I'm sure with the constant upgrades being performed on aircraft nav systems, cell phones SHOULDN'T be a factor these days. The older 1st generation nav systems found on the older model 727s, 737s, L1011s, and so forth are more susceptible to interference by OLDER cell phones. The newer phones should not be too much of a problem, unless you have a plane full of people using their cell phones all at the same time. I can only imagine what a plane would look like after that. Imagine an eerie greenish glow around the plane as it lands.
Happy flying!