Here's Nvidia's official statement, along with an extra warning from Blizzard:
We are aware that some customers have reported fan speed issues with the latest 196.75 WHQL drivers on NVIDIA.com. Until we can verify and root cause this issue, we recommend that customers stay with, or return to 196.21 WHQL drivers. Release 196.75 drivers have been temporarily removed from our Web site in the meantime.-Nvidia PR
We're getting reports where users are getting intermittent low FPS after installing these drivers. It seems that it is related to the fan control included in these drivers not working correctly and is causing the video card to overheat on 3D applications. This will affect Warcraft 3, World of Warcraft and StarCraft 2 Beta. Please uninstall the drivers and revert back to the older ones.