John -- take a look at open shoe
OpenShoe

3d motion sensor (numbers a crunched, no post processing)
3D Motion click — board with MM715 9-axis motion module | MikroElektronika

Excellent Google Talk on Sensor Fusion from 2010 (explains the
need for number crunching from sensor)
Sensor Fusion on Android Devices: A Revolution in Motion Processing
Sensor Fusion on Android Devices: A Revolution in Motion Processing - YouTube

Andy Grove discussing big data in 2012 (motion tracking for Parkinson's)
Wired Health Conference: Andy Grove - YouTube