Demonstration of remote keyboard sniffing using an antenna: http://www.vimeo.com/2007855?pg=embed&sec=2007855

I've heard that passwords can be compromised by the sound (probably over multiple entries) of the keystrokes, since the attacker can figure out how far apart the keys are and listen for patterns, though I haven't seen any demonstration of this yet.

Each key also sounds different. One of their attacks uses this, in a slightly extended sense.

Amazing! Now all we need to do is convince potential victims to use PS2 keyboards and we're golden.