When airborne you are in line of sight with too many GSM masts and I've heard that this may cause problems because your GSM is trying to contact too many masts at once. Overlapping frequencies, that sort of thing. Apparently to the point where you might bring a whole area down.
That is unlikely... What would happen then is that you lower the capacity a tiny bit in the affected cells, which probably won't have a noticable effect for anyone, and that you might lower your own quality of service. If airborne cell phones were more common, cell phone networks would have to be planned based on that, which would require more radio resources for a given area. Therefore, networks operators wouldn't
want you to use them in the air. But when it's just a tiny number of GA pilots doing that, I can't imagine this having any effect at all on the cell phone networks.
More modern systems (increasingly sophisticated WCDMA releases, and eventually LTE) are even more resilient to such interference, due to more sophisticated resource scheduling algorithms.
So not a problem there!
