They will never swap it, because they probably won't be able to reproduce the problem in the shop (no signal), and anyway the GPS in my Ipad2 works perfectly on the ground i.e. when not in motion.
I have taken a few things back to the "Genius Bar" (we have had a 100% failure rate on Apple products - admittedly based on just 2 devices
so a very small sample) and while they give the appearance of being very helpful they will skilfully fob you off unless they can reproduce the problem.
Even then sometimes not. I took back an Ipad2 with the Apple HDMI adaptor, which was producing a "accessory not recognised" error message if the Ipad was turned on with the adaptor plugged in. Apple refused to touch it, saying the error message wasn't important. In fact the HDMI output was flakey, sometimes working with some apps and not others, and we have given up on trying to get it to work.