Surely there are lots of possible solutions -- tapping multiple points of a photo in sequence (am I the only one who prefers tapping for the reduction in finger prints?), camera face recognition, etc. You could even use a physical gesture, detecting patterns of movement of the phone through the accelerometer and gyro. Hmm... maybe I need to patent that!
A physical button would have been fine too -- something like the mute/orientation button would be enough to prevent most accidental triggering. Not very discoverable, though.
Right now my the only method I actually like is the iPad smart cover, as it's already unlocked by the time I have opened it. I tend to use a flip open case for my iPhone -- to protect from the inevitable fluff while in my pocket and to help the speaker when open (angle it just right for a big improvement in volume) -- so I'd like to see the iPhone 5 include that magnetic detection technology. Not very likely, I know.