AMD have reportedly cut down the AIB allocations for their Radeon 6950 range. Supposedly the 6950 board allocations were cut and possibly delayed from actual shipment.
While this news is not 100% accurate, this is the leaked information we have. The 'word on the street' is that there was a last minute BIOS tweak (possibly to bump speeds or something, maybe to combat the GTX5x0 cards in the wild). Keep in mind that this is not unusual for product introductions - another thing to keep in mind is that there have been zero reports of any hardware problems on any of the Cayman boards.
Of course, there is a rainbow at the end of this story - allocations for 6970 boards just went up considerably.