i sat next to a pilot once on a flight and asked him about the cellphone thing, what problems they could cause. he told me that the only time it would interfere would be during an instrument landing...at that point it could cause a crash. the odds are against it causing a problem, but obviously those are odds you wouldn't want to be on the wrong side of.

(that said, i've had my cell on in a checked bag as well. i realized it was on before the flight and told the ga. they sent someone to locate my bag and turn it off.)