Is image stabilization useful when shooting from a flying plane or helicopter?
Yes, it can help minimize camera shake. To take it to the extreme, serious aerial photographers sometimes use a gyroscope mount.
Related but not directly answering the question: you want to ensure that the camera is not touching the airplane or helicopter - you'll end up transferring all sorts of vibration from the aircraft to the camera.
This article about Nikon VR suggests that not all forms of VR are best when taking photographs from a helicopter. Search for the word "helicopter" and note that it suggests you use "Active VR".
Basically, Active and Normal VR help with two different kinds of camera-shake. The former helps with camera-shake due to vibrations in your surroundings while the latter helps with unsteady hands. The lens uses a different algorithm based on the pattern of vibration in these different situations to stabilize the image.
Caveats (to my understanding):
- The article is talking about using fast shutter speeds
- The article is really focused on Nikons -- I do not know if there is a Canon equivalent of "Active VR"
- The author of the article admits that there is controversy over the article and he doesn't seem to have any numbers or examples to back up his claims. That being said, he seems to know what he is talking about and I don't think it hurts to apply his techniques and see if you get better results :-).
Hope this helps.