I flashed mine with success but I bought in a while ago. I did a lot of research and I mean a lot. You should know that what you get is not a 6970. It is a 6950 with increased shadders which does give a performance increase in some video areas. There are other differences between the cards besides the bios/shadders. I did the flash because I wanted to play around with it. I will advise you not to do it unless you are 100% willing to accept bricking to card and having to trash it. It has happened to many people. For my clients (I build custom rigs on a limited basis) I always advise that if you want a more capable GPU buy the next model up. As to how to do it; I will give you the basics and you need to research and learn how to do it. It will be your responsibility if it does not g right. So to do it you first make a copy of your cards bios. Do not use any bios versions available on the web or whatever. This is essential. Then you modify the bios copy activating the additional shadder option. Then you flash the modified copy onto your card. Three simple steps, copy, modify, and flash. Remember you will not have a 6970, you will have a hybrid version of a 6950.
If you have recently bought the 6950, there is a chance that the card wont even be able to do it. Me and DogSnake bought our cards awhile ago. It was before they started laser-cutting the shaders off the core.
You cannot unlock shaders that are not there basically. I heard this was true for the newer cards, but I haven't been sure.