An orientation sensor is not an accelerometer, and I think just about all the Powershots have it...
You're right, too, but I can't honestly see that approach working too well...
(Edit) To elaborate on that, there's three main issues I see with using the viewport.
The first is that you often won't know how to interpret the data: what if the light is so low your algorithm can't really see much of anything? What if there is no clearly discernible horizon in the landscape? Ouch.
The second issue is that the algorithm would really have to be clever and fast to do the guessing right! Unless there's something I don't know, we only have a little ARM processor to work with, and with that, we'd need to find a way to make an accurate guess as to where the horizon is, in 30 milliseconds' time. Ouch.
And finally, even if we can do that guessing constantly in real time, just how much battery power would that drain? Ouch.
