Almost all of the digital meters have an IR LED that blinks once for every kilowatthour that is consumed by the house. Their device (along with the one that Black and Decker sells) sits on top of the meter, counts the number of blinks every few seconds, then sends that over some sort of wireless to the internet.

I have yet to figure out how to intercept the Black and Decker's signal to duplicate wattvision's functionality. This is easy(ish) to do with a mini Arduino and a pair of XBee radios.