We got some official data about AMD's Radeon HD 6970 card codenamed Cayman XT. The card runs at 880MHz and the card has the memory that runs at quite impressive 5.5GBps. The memory of choice is GDDR5 in 256-bit flavour and the card has 2GB of it.
The card has 96 texture units, as well as 24 SIMD and 1536 ALU streaming processors. It has 32 ROPs and 128 Z stencils.
Cayman Radeon HD 6970 uses VLIW4 Shader Architecture something that Bobo explain in details here. We can confirm this story today.
Total compute power of Cayman in this version is 2.7 TeraFlops under IEEE754-SP standard and 675 GigaFlops with IEEE754-DP standard.
It is unknown if the card is slower of faster than the GTX 580 but this given presentation didn't really look into it. The indication is that Cayman won't be faster than GF110 products, but in this presentation, ATI didn't show any Radeon HD 6970 performance data.