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A cell phone should not interfere with avionics installed in an aircraft; however the noise caused by these can be distraction.
The FAA and NASA (through its ASRS Program) has reviewed the issue and did not find a single instance of cellphone interference with avionics. Since noise is being reported, this would suggest that if noise were reduced, there should be no safety issue.
When an airborne relay or base station is provided, the issue with cell search and strong pilot signals would be alleviated.
A separate GPS page describes aircraft VHF NAV/COM radios interference with "suction cup" GPS receivers. These are not installed avionics, but are GPS receivers in which the antenna is attached to the window in the manner of a radar detector. These GPS receivers have no problem if an external antenna is mounted outside the cabin, and the issue is totally irrelevant to installed GPS NAV systems.
There is an economic incentive; that being that the airlines will want to devise a way to control the use of in-flight cellphones in order to impose fees on passengers. Presently cellphones are the major dollar drain on airlines. Expect "for fee" entertainment to be impacted when pax start using portable DVDs.
Expect more stringent restrictions when advertising becomes a major economical factor in in-flight entertainment.
The major safety exception exception is one of precaution. This exception is the "10,000 ft." rule in which computers are to be turned off below 10,000 ft. (3000 M but flight levels are based in feet. 10,000 ft. is Flight Level 100 except in North America, where it would be Flight Level 10 but for the fact that we don't use flight levels below 18,000 ft.) This is precautionary. I would not expect a computer would interfere with installed navigation in an aircraft, and I have seen installations in which a laptop computer is coupled to aircraft navigation systems for moving map functions in private aircraft.
The logic to the "10,000 ft." rule is that if anything does go wrong in IMC, there may not be sufficient time to maintain aircraft separation. It is common practice in aviation to provide for such redundant safety procedures, and "turning off the stuff" fits within this model.
Analog cell phones (the old "brick phones" and "bag
phones") will jam multiple cell towers (cells) when used from
the air. This type of jamming was obvious enough to be immediately
recognized by cellphone operating technicians. This specifically
does not happen with CDMA, and I have seen no reported problems
with other digital technologies.
See the separate GPS page for information on handheld GPS units and frequencies which affect handheld GPS units.