NVIDIA has upset many mobile gamers by disabling overclocking on their mobile GPUs in the GTX 900M series in a recent driver update. Under pressure from any angry users NVIDIA plans to revert this change and bring overclocking back to mobile GPUs.
For those who are wishing to overclock right now (to make certain games and settings playable again), NVIDIA suggests users revert back to the older GeForce 344.75 drivers.
Source: TechPowerUp! via GeForce Forums | News Archive