There's a bit of a misconception about the whole unlocking thing. Let's go back to December, when AMD had to release the 69xx series, as the Geforce 5xx series had been launched to very positive reviews. The 570 beat the 6870 and 5870 comfortably enough and the 580 was untouchable.
AMD had to respond and had to release the 6950 and 6970 in time for christmas, which is a critical sales period in the US and other developed markets. They had issues with production of 6950 and 6970 PCBs, and in order to meet the deadline, they decided to release both 6950 and 6970s with the 6970 PCB, the only difference being that the 6950 models would be clocked lower, with lower grade memory to cut costs and the number of shaders would be reduced to 1408 in the Bios, to artificially create a performance gap. Now why they decided to go the dual bios route is still not clear..
Once the cards were released and the reviews started coming in, some people noticed that the performance difference between the two cards was almost too perfect around 10% consistently, in title after title. And physically, the cards were identical.
So some guy decided to slap on a 6970 Bios, just to see if he could unlock it. He did and the news burst out on the internet and boosted the 69xx sales.
AMD meanwhile, sorted out their production issues around March, and now had enough supply of 6950 and 6970 PCBs to sell to the market. The new batch of 6950 PCBs were manufactured with 1408 shaders from the start and thus, there are no hidden shaders to unlock anymore.
The flurry of unlocks happened in the initial months, I would say between December to March in the US, and from January to May everywhere else, with reference PCBs with a dual bios switch, 2 GB Vram and the stock cooler on the right side. But now, since AMD was aware that unlocking had given them a lot of word of mouth publicity on the internet, they had to release random 6970 PCBs in the guise of a 6950, to keep the buzz alive and continue the strong sales..
The majority of 1GB 6950 models are the new PCBs, with a central cooler, and unlocking on these is very rare. Almost all the new 6950s, even with 2GB Vram, are custom cooled, with redesigned PCBs which in most cases only have 1408 shaders. Any new unlocks you hear about from now on, will be entirely due to luck with the random 6970 PCB I talked about earlier.
Unlocking the shaders themselves gives you a very small boost, and only when combined with an OC to 6970 clocks do you get a boost.
Sorry for the long post
