It's amazing what today's professional overclockers can get out of hardware nowadays. A day before the GTX 680's release, famous overclocker Kingp|n has managed to hit an incredible 1842MHz using LN2 and a modified PWM section, shown below and the below at a chilly minus 150C. This was done at EVGA's Taiwan R&D center.
For the rig, the CPU used was the Core i7-3930K at 5.5GHz with system RAM at 2400MHz and the latest Precision X overclocking utility. The combination gave a 3DMark 11 score of 14912 on the performance preset. It was narrowly beaten by the Elmor's HD 7970 at 1800MHz which scored P15035, but this test was done with full tessellation. It would have been almost 1000 points higher otherwise.
Kingp|n said the following on his forum:
Setup is capable of 16k single card regardless when pushed all the way and today we got 1900 stable, we think 2ghz is possible with more tuning. Just need more time. Tried 2 cards on Ln2 to obtain this result so far...its all modding that makes this frequency possible.