this can be done with signals sent through your monitors cable as well.... to get a birds eye view of whats on your monitor... (demonstrations are available online... google is your friend tho cos i cant think of any links)
dabees may have heard the term 'really free intelligence' as a joke acronym for RFI..... (radio frequency interference)
also... old computers 'the kind that filled a room' could be made to play music (albeit crude music) by holding an AM radio next to the core memory unit and rapidly changing segments of it... (or in other words changing the contents of system ram at specific speeds and patterns to create musical RFI)
wanna snoop on yourself and have a portable AM radio handy? tune the radio to a place on the 'dial' that doesnt have a station on it.... now put it near your keyboard cable and type.... put it near your monitor to hear its noise (this is especially vibrant noise if its a CRT) or near the cat5 cable running between your comp and the router and start... then stop... a download or upload... notice anything?
as an off topic to forested; turning off my CRT monitors actually let me listen to the shortwave band... before it was full of unusual noise that didnt dawn on me what it was till the antenna fell back and actually touched the monitor.... was definitly a 'oh... duh!' moment....