I think the iphone 3g and 3gs will indeed get gps without a cell tower. I have had perfect gps in the mountains, where the is no signal listed at all. Only iphones with built in gps though, not the original iphone which only had skyhook "gps" approximation.

that being said, I have had only hit and miss sucess with gps on a plane. My iphone didn't get it, although I wasn't at the window.

My htc mogul didn't get it, even when I was AT the window. My garmin colorado DID get it. And my garmin xy400 watch, which was on my arm at the time and up against the window GOT a signal. Fun to watch my running speed at 400 mph