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.
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.
here is a wikipedia article about it:
http://en.wikipedia.org/wiki/Power_analysis
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).
http://www.defcon.org/images/defcon-17/dc-17-presentations/d...
EDIT: sorry, that PDF is less informative than the talk was. He had a bunch of demonstration material that's not included here, apparently.