I'm sure some product line manager at AMD has noticed the strange selling out of 6990s and 5970s, and if they have even an ounce of grey matter they've deduced it's the result of Bitcoin. But to support the development of a custom ASIC by one of the big boys, the card sales volume needs to be truly astronomical. I don't see Bitcoin really changing the course of NVIDIA or AMD until there can be provable demand for millions of such custom cards.
In the mean time, there is definitely a niche market for a custom Bitcoin ASIC, if someone can get the job done profitably.
I think you vastly underestimate the number of non-bitcoin users of these cards. At best, bitcoin would be lumped in with the other distributed computing projects that can use GPUs. And don't forget the armies of actual gamers who purchase these cards. Bitcoin, although large, is very much just a drop in the bucket.
Also, you will not see a bitcoin ASIC. Complex ASICs required highly skilled (read: expensive) engineers and even more expensive processes to put them into place. When there is already a very good solution to the problem (GPUs) and an enormously volatile and unpredictable exchange rate, you're not going to see such an investment. Not to mention that investors in such a project would almost certainly demand immediate cashing out of mining proceeds to pay off their investment, which would put significant downward pressure on the exchange rate. ASIC mining is a pipe dream.