I use GPS all the time -- people look at me funny sometimes but I've never been told to turn it off. I find it fascinating to know the exact route and velocity and altitude all the time!

GPS recievers are exactly that -- recievers. So yes, it does give off interference by virtue of being turned on, but it is not radiating anything beyond the normal electromagnetic noise of any small electronic device like a CD player or video game.

In fact it should give off considerably LESS noise than a CD player since motors (like those in CD players for spinning the disc) are incredibly noisy. Heck, a laptop computer with CD drive, hard drive, and dozens of chips probably gives off a hundred times more EM noise than a handheld GPS.

If you really want to worry someone, be sure to tell them that the plane is being bombarded by the GPS radio signals from every angle during the entire flight! Those pesky satellites don't stop transmitting just because a plane is taking off or landing...