Somewhat relevant: My brother attended a conference where a speaker said that gait of human beings are so unique that an individual can be identified with 100% certainty using the accelerometer of the phone in their pocket.
EDIT: Found a paper on the subject from 2009: http://www.cs.yale.edu/homes/mfn3/pub/mfn_gait_id.pdf
EDIT 2: Found another paper from 2012 claiming 99.4% accuracy: http://users.ece.cmu.edu/~juefeix/btas_2012_felix.pdf