There used to be a mode to geo-fence data collection, i.e. harvest only outside of a certain area, but I don't see it anymore.
We decided to cut out geofencing because it was confusing to most people and reducing the number of knobs and dials on a program is usually a good thing.
We've got a bug filed to hook the accelerometers though: https://github.com/mozilla/MozStumbler/issues/1107