I heard a rumor they were going to disable that on 6950's mfgrd after January, but I have no proof other than remembering an article online...I dont remember what site it was even on...but I myself wouldn't buy one planning to unlock it at this point in time. Sorry for the lack of anything concrete, but I figured any reply would be better than none after one empty bump already :-/
Not all cards are the same, and it depends how lucky you are. the older ones are easy to unlock, newer models, even though identical, may have problems. Remember that only the 2 GB model is unlockable. When the first 69xx GPUs were created, they had a high clockable rate, meaning they could all run at 6970 clocks, but couldn't be as AMD wanted something cheaper, thus 6970s were rebranded as 6950s and were deprived of one 8-pin PCIe and given a second 6-pin PCIe. Newer models may not be unlockable, so get a brand that is not heavily bought, and get reference card, not an MSI TwinFrozr. Basicly its overclocking, some CPUs overclock beautifully to 4 GHz, others fail to reach 3 GHz.