I'll have to dig it up (maybe if I get time tomorrow I'll try), but I've seen a couple research papers where they wrote software that got was pretty effective a

I'll have to dig it up (maybe if I get time tomorrow I'll try), but I've seen a couple research papers where they wrote software that got was pretty effective at decoding peoples passwords by listening to them type. Between different keys sounding slightly different, the timing between keystrokes, and an individual's personal typing patterns, it's not even all that difficult. So yes, that sort of thing has definitely been done.