Nvidia has listened to feedback and reenabling overclocking on the GTX 900m.
If you want to reenable it immediately, you can revert to driver version 344.75, but an upcoming update will bring it back permanently.
Nvidia shared this message to consumers:
As you know, we are constantly tuning and optimizing the performance of your GeForce PC.
We obsess over every possible optimization so that you can enjoy a perfectly stable machine that balances game, thermal, power, and acoustic performance.
Still, many of you enjoy pushing the system even further with overclocking.
Our recent driver update disabled overclocking on some GTX notebooks. We heard from many of you that you would like this feature enabled again. So, we will again be enabling overclocking in our upcoming driver release next month for those affected notebooks.