Actually Wizzer, I can't agree with the statement that previous generations before the 600 series had fixed voltages.
To prove my point I took a single GTX 580 card, and set it for plain vanilla stock operation. If I let it idle long enough, the voltage will drop to 890mv @ 50MHz. Normally the quick drop after a few seconds of no activity is 963mv @ 50MHz. At it's full rated Core clock of 797MHz, the voltage goes up to 1075mv as you can see in the attached screen shots. If I have the card over clocked to 920MHz, the voltage will go up to 1150mv, and then back down to 890mv @ 50MHz.
Unless I'm missing something, it appears that the same protocol is in place for the 500 series cards as well. I should point out however, that this only works in this fashion when the Nvidia Control Panel settings for the driver is set to "Adaptive". If I set it to "High Performance", the cards will stay at 797MHz and 1075mv all the time.
Therefore, I have concluded that this is a function of the driver settings and not your fancy, faster then mine, 680 cards!