The story begins with a thread started by K404 in the HWBOT forums, about six months ago. Kenny noticed any core clock frequency over 999MHz would not result in any performance gain, even though GPU-Z would report higher frequencies. Over the course the thread, more users found this little bug to be hurting their overclocking experience as they were forced to go back to older and less performant drivers. In the beginning of this month, HWBOT staff member Christian Ney contacted Nvidia with the request to fix this bug. About twenty (20) days later, we found this in the release notes of the latest 320.18 driver release:

It seems that the clock frequency limitation has finally been resolved. So far, we are still waiting for the first users to confirm this. If you have a Fermi card and found the fix to be working, feel free to post in our forum thread. You can download the drivers and read more about this driver release at GeForce.com

Thanks Nvidia!