[tezza]

And would you know if they just recorded the keys your fingers pressed?

I think Jakob is really onto something here.

[pygy]

They could record the sound of the keypresses too. They may be impossible to discriminate subjectively, but they all have a distinct signature...

[mgj]

Yep, see: http://www.cs.berkeley.edu/~zf/papers/keyboard-ccs05.pdf