Blue's News, among others, report that Nvidia is recalling the graphics card driver update it issued on March 2. Alarmingly, a problem with fan control in the firmware can result in card overheating, potentially damaging the graphics card or computer.
If you downloaded and installed Nvidia's ForceWare 196.75, you're encouraged to revert back to version 196.21, which you can find via Nvidia's driver download page here. Nvidia has acknowledged the issue and pulled the 196.75 version while it tracks down the problem.
Update (March 17, 2009): Nvidia has responded with updated drivers. Additional information.