Definitely a matter of when not if.
I've never programmed for Symbian before, I guess this is the reason why the n900 is the perfect platform for this kind of thing. The full Linux system makes it really easy.
Although a light mobile phone does seem to be pretty well suited to flight instrumentation, if one could get discrete components to communicate with a base station (perhaps the n900 itself!) it would probably be a lot cheaper when it crashes, and probably lighter in the air. In terms of getting the data to the user, a transparent hud seems like a good middle ground between regular flying and fpv flight.

I wonder how tricky it would be to get a hud system like that working; again, all the hardware is here, just easy software