VehicleAimGetAngle() to replace UnitPitch()
Fix dragging (left click + alt)
Working fine in MoP
Now provides proper pitch for unit instead of camera angle.
Thanks to akazik for pointing out the function addition (came in 3.0) and thanks to Blizz for adding it 
A very simple mod that displays your current pitch. It is meant for mouse flyers so they can see their current pitch (+ve is angled up)
Alt+left click to move
No idea on its impact on CPU usage
Only display in flight
For those of you looking for a more direct solution to flying straight you might consider levelflight
There is also a much more aircraft looking version FlyPitch
Sadly, I don't seem to be able to restore the original addon I hadn't updated since 7.0 :(, so this is a recreation