Drawing 2D pattern is the same slide, just path is more complicated.
As for popularity, I think it's because that's pretty good solution: you have to lock the screen do disable accidental manipulation. Simple tap doesn't work in this case, physical buttons are also less than optimal.
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.
