Tightslot,
Are you being serious?
Are portable GPS devices not sold specifically for use as aircraft navigation?
I've been a professional airline pilot for almost 20 years. I've used many such devices. I know my passengers have as well.
This whole idea of interference from a GPS which only receives signals (no transmitting capability whatsoever) is ridiculous.
These things ARE USED FOR AIRBORNE NAVIGATION. LEGALLY! Get in practically any general aviation aircraft and you'll see that. They are built for aviation use.
I don't think a passenger switching on the same thing on a commercial flight is going to change things up.