I've seen dedicated gps systems pick up on satelites and get locks when in planes. It is possible that the N900 just isn't as capable GPS-wise, what with being filled with a bunch of other hardware.

@Fixfox: I think mp-fremantle-generic-pr IS part of the Ovi Maps application. It just isn't as clearly titled. I'm by no means an expert, but just letting you know, I think it's safe to remove it.