Apart from fingerprinting that has already been mentioned, a relevant article: “ACCessory: Password Inference using Accelerometers on Smartphones” http://www.hotmobile.org/2012/papers/HotMobile12-final42.pdf

“We show that accelerometer readings are a powerful side channel that can be used to extract entire sequences of entered text on a smart- phone touchscreen keyboard.”

Pending research to the contrary, I would assume that what is typed on the keyboard of a laptop can be recovered through the accelerometer by an application running in the background.

Fingerprinting devices for one http://blog.sfgate.com/techchron/2013/10/10/stanford-researc...

I wonder how many bits of entropy that fingerprint has, though. 8 bits would make for an impressive and scary looking demo, for example, but for ad tracking it would be useless.

I don't think ad tracking is the scary application...

That's a good point, and p = 1/256 is definitely good enough to convince a judge or jury.

It's not unthinkable that accelerometer data might reveal more information about a user than cursor motion and keyboard input. Home layout, restroom/kitchen/etc. habits, body type, chair type, and even the squishiness of one's clothing could conceivably be determined.

Is this -currently- the case? Highly unlikely. But will someone seek to profit from these data in the future? Probably.

Using a privacy prompt for access to any sort of sensor seems like a reasonable policy to me, keyboard/mouse/touchscreen excluded.