Many issues arise when using different freqs of communications. I had to move a KU antenna the other day because of FM interference. No more elaboration possible on that note though
Just because a cell phone does not use the same freq does not mean it an not be screwed up ue to overlappig harmonics and noise interference. If a cell phone being on reduced the effectiveness of a planes radar by one percent, the FAA would ban them for pure ease.
Also to think of is the range of cell towers, which if memory serves is in the 5 mile range. This is how high planes fly anyway, which would reduce a solid coverage map to a plane with nodes of service only over each tower, even less if the towers are directional(read: less if sprint does not like to waste money).
if a person wanted to try and sneak a battery aboard, I would suggest a Xstatic Batcap. Give them a look, esp at their juice per pound.
want an antenna that could go on a plan without making it a U2? look at the 360* antenna from Inmarsat. a large salad bowl is all you need
no links, I am lazy
for more info, someone should ask MCL he knows a ton about this stuff.