It's not just the raw sales. As other people mentioned, it might be 1-3% boost. Just looking at some of the top online retailers, it's extremely hard to get quite a few of the ati models and nvidia from what I've seen doesn't have any stock issues. It might be 1-3% so it's not a game changer.
If people want the old generation 5xxx cards there is a problem in getting stock (obviously), but in Australia we have no problem getting as many 6950, 6970, and 6990 cards as required. Is Australia siphoning all the stock headed for the USA?
Bitcoin is barely a blip on ATI's radar. They sell far more to the OEM market. Second is the enthusiast market for gaming. Then comes the Stream (and CUDA for nVidia) market for scientific computing. Bitcoin brings up the rear. The current hash rate is equivalent to 36,000 non overclocked 6950 cards. I'm sure ATI ships millions of GPUs per year. A few tens of thousands isn't worth worrying about, especially for something like Bitcoin where users may lose interest.