I'm intrigued by this. Most of my mobile typing is done with the Google keyboard swipe gesture thing. Is there a way to identify a user by this, since the words are entered in the form field one at a time? Also, accelerometer data as in the way the use holds the phone? I find it interesting because for the past week or so I've been trying to fix my bad posture, and as a result the way I hold my phone has changed drastically. I wonder how this type of system deals with situations like that.