Samsung screws up GDDR4 yields
 

The problem with this is the upcoming high end GPUs need these parts, and if availability is limited, so may the selection of GPUs on the market.

ATI has a plan B. You may see some cards ship with GDDR3 instead of GDDR4. As long as the high end parts have GDDR4, the lower end parts can probably make it with GDDR3 and not take much if any hit.