I found my car eventually in a big hole, apparently the altimeter works to +/- 25ft accuracy. Although I do like the sound of +600mph!!
Mark, the GPS doesn't seem to get too confused in trees. It worked pretty well in Dalby when i was last there. There is a speed/distance sensor (and cadence) included with it, i just didn't see the point on a full sus as it mounts to the crank arm and chain stay.
The Garmin Edge does a bit, but generally it'll just autopause until it gets the signal back. If you have the speed/cadence sensor it doesn't autopause but will simply leave a gap where it knows you're moving but isn't sure where until it regains a signal! I only use it on the road as a glorified bike computer (useful for the data save'n'download capability) - I'm not sure how trees would affect the use for navigation.