The practice of reducing clock speeds when sitting at the desktop in order to conserve power is standard fare for mobile graphics processors. However, desktop graphics cards seem to do it less consistently—if at all. According to Expreview, the latest GeForce desktop graphics cards don't throttle when idling, but Nvidia will change that with its GeForce 9500 GT.
The Chinese site says it discovered clock throttling behavior while testing the GeForce 9500 GT. Using ForceWare 174 drivers, the card's clock speed reportedly dropped from around 600MHz to 200MHz, taking GPU temperature down from 55°C to 50°C in the process (and, we'd wager, lowering the fan speed, although Expreview doesn't mention that).
Judging by earlier rumors about the card, this GeForce 9500 GT will be based on a new G96 graphics processor, and it will have a default clock speed of 550MHz, 512MB of 500MHz DDR2 memory, and a 128-bit memory bus. The card is expected to be a replacement for the GeForce 8500 GT, so it may launch at less than $100.