It's true. It's something I've noticed in a few games, and its especially noticable to me because Fraps outputs an FPS display onto my G15 keyboard's display. It was most noticable in Bioshock, where the FPS while I was afk REALLY looked like a heartbeat, with the double pulse and so on. Anyways, I finally screenshot'd it. It's quite intriguing. My belief is, given that it happens after I'm afk for a while, and the FPS trace is perfectly formed (thus not influence by external factors, such as the environment complexity), it's some powersaving feature. Fixing the FPS at 30fps must require less processing, thus less power. It's interesting nonetheless. This is the FPS trace from Guild Wars, in a fixed environment, AFK, with nothing changing on the screen. I had the UI turned off so chatlog changes wouldn't affect the updates in anyway.

The flatline is 30fps, the peak of the pulse is 40fps. And the time period between pulses is five seconds.