Information Leakage from Optical Emanations [PDF].

A previously unknown form of compromising emanations has been discovered. LED status indicators on data communication equipment, under certain conditions, are shown to carry a modulated optical signal that is significantly correlated with information being processed by the device. Physical access is not required; the attacker gains access to all data going through the decide, including plaintext in the case of data encryption systems. Experiments show that it is possible to intercept data under realistic conditions at a considerable distance. Many different sorts of devices, including modems and Internet Protocol routers, were found to be vulnerable.

There's a similar risk (see also this paper [PDF]) for CRT monitors giving away their data, even with diffused reflected light off a wall. (Thanks memepool.)