Link is out of date...
From what I have heard, AMD indeed did something to the cards physically to prevent this. I have also heard that some people are still having success flashing the bios, and having 6970 results...maybe depends on the binning of the cards?
I can tell you that I have a HD6950 purchased about 5 months ago that I was able to flash to 6970. It is a Diamond HD6950 2Gig version.
CJ