The hack I'm referring to was somewhere in the 70's or 80's, I heard about it in the 80's (86 or so).
I'm sure it's been done plenty of times though, not just recently. The nasty thing about it is that such a leak can be in place for a long time before it is discovered.
Which states that it was introduced in '98, but I'm quite sure of when I heard about it because I remember who told me (a systems programmer for a bank that I worked for in those years).
There was another presentation at Defcon about attacking USB drivers with rogue devices. Basically, they programmed a USB-enabled microcontroller to present a malformed ID string and could use it to inject and run arbitrary code.
It mentions this idea of modulating power usage, and a few other clever ones, though I didn't see a reference to it being used in the past.