Your suggestion is spot-on. I have done some searching and would like to point out a couple of things:
not everyone uses Safari and/or iPhone -- in fact Android has more market share.
Android apparently doesn't support accelerometer events via Javascript -- or didn't
And thanks for the mozilla docs, but I would wager only a very small percentage of mobile phone users actually bother to install firefox on their phone.
Sounds to me like depending on Javascript detection of orientation is probably not the best idea. Thoughts?
EDIT: After a little more searching, it would seem that Safari usage exceeds Android browsers significantly, despite Android phones having more market share. I expect this is due to the head start the iPhone had on Android.