unless you're flying a glider or light sport aircraft, keep your GPS off.
That goes against the practice of the vast majority of GA in this country. GPS is used (even in the handheld type) a lot in Group A aircraft alongside all the other VOR/ILS/DME/ADF stuff in the cockpit. Mobile phones interfering with the DME is a completely different issue though.
I asked the question not because I want to use GPS during a commercial flight but because if it has the potential to interfere with airliner systems it has the potential to interfere with the avionics in my plane as well...which is very worrying.