Thanks Guys - Price is a factor (of course), so GPS will probably be out. Rsocor01, you have a point about the "jarring" effect of walking and the use of an accelerometer to detect that, but now we are getting back to vibration... Not sure how reliable that will be - might still be an option. My dream sensor is something that could be aimed at the ground and can determine when what it is looking at has changed - as what happens when an optical mouse moves. If I could hold an optical mouse 3 feet off the surface and still have it detect movement, that would be ideal, but we all know that won't work...