This a review of a paper by some reasearchers who have developed a method of translating the sound of keystokes into text. The method requires only fifteen minutes of recorded material to be useful. Apparently passwords are somewhat difficult because the program is designed to take advantage of characteristics of English, but it can produce a short list, and as it says, one which usually includes the actual key. Presumeably if the program was modified to take into account period of time around keystrokes, picking up passwords would be much easier.
Acoustic Snooping on Typed Information
Friday September 9, 2005 by Edward W. Felten