Probably the acceleration vector. If the phone is rapidly moved a meter away from you, either it's being snatched or it's being thrown.
Edit: To clarify, I was thinking of horizontally, in the direction that corresponds to the top of the screen, as if you were bent over using the phone--probably holding the bottom-of-screen--and then someone grabbed the top-of-screen to pull it away.
A drop registers as no acceleration while in freefall, and then a sharp spike when it hits the ground. This was counter-intuitive to me when I first figured out how to display my phone's accelerometer readouts, but makes sense.
I think a lot of the false-positive cases where the screen gets locked are acceptable in context.
I mean, most people dropping their phone will be too glad/devastated that the device did/didn't escape harm to bother being annoyed that they have to unlock the screen again.
Edit: To clarify, I was thinking of horizontally, in the direction that corresponds to the top of the screen, as if you were bent over using the phone--probably holding the bottom-of-screen--and then someone grabbed the top-of-screen to pull it away.