In the early days of desktop PCs, the CPU was essentially running at full load all the time (in an idle polling loop) and the temperature would stabilise after a warm-up period, so this type of attack wouldn't be possible. This was when processors only consumed a few watts. Now they vary between a few watts at idle and 100+ at full load, and can make such power transitions in a few microseconds.
I think the acoustic method is more clever (and has a higher bandwidth):