According to FUD Zilla, the strong performance of the GeForce GTX 580 caught AMD off guard. The site claims the original Cayman was not fast enough to beat the GeForce GTX 580, prompting AMD to delay the launch in order to make the card more competitive.
The dual-GPU Radeon HD 6990 "Antilles" graphics card on the other hand has slipped into 2011, and it looks like NVIDIA will also introduce a new dual-GPU part in the new year.