I've seen some fun papers of 'Well, you could do this awful thing.' (comparison of accelerometer data to deconflict which nearby phones are in the same vehicle vs separate ones to better refine social graphs), in addition to all the stuff we know is being done (ultrasonic signals in various ads, tracking shoppers by their wifi/bt beacon MACs, etc). At this point, the 'sneaky snacky smartphone' approach to data collection (in which everything that can be collected is being collected, and probably used for things you can't imagine it would be useful for) starts to press heavily on the 'And I therefore shouldn't carry a smartphone' side of the scales.
