The HD 69970 should ship in decent quantities and I also saw that Radeon HD 6950 should also launch on the 15th, but in somewhat lesser quantities.
Sources indicate that Geforce GTX 580 should end up faster than Radeon HD 6970 at least in many of the popular benchmarks. The fact that AMD is pricing Cayman cards a bit lower than the 580 lends a bit more credibility to such claims. I have also heard that driver was one of the reasons of this “slight” delay of Cayman launch, and I guess that part of the story is that Geforce GTX 580 ended up much better than ATI expected.
In the GPU world it is very easy to slip the schedule and it can involve all kinds of issues, from poor yields, bad beta driver, to a shortage of some components. At least we know that the card should be in some stores on December 15th.
I will update you with more information especially on price and performance.