Ed Felten blogs how learning machines can deduce text by listening to keystrokes. Another reason to get a quiet keyboard….
One interesting point: Somewhat counterintuitively, with this approach a longer password is easier to guess than a short one – though, of course, a non-sensical password will be hard, since the approach relies on character counting.
Reference: Li Zhuang, Feng Zhou, J. D. Tygar (2005), Keyboard Acoustic Emanations Revisited, University of California, Berkeley