A Radeon 6990 has 4 gigabytes of ram.
If the task is "find a number that bcrypts/scrypts to less than a given hash target," I don't see anything that would stop a GPU programmer from implementing bcrypt/scrypt on the CPU and parallelizing at the try-different-nonces level.
Maybe I'm missing something; I'm probably biased because I worked at SGI from 1988 to 1996 and saw first-hand the evolution of GPUs from very-special-purpose chips with very limited memory to very-general-purpose vector-processing pipelines with very fast access to lots of memory.
HD6990 has more than 3000 processing units so bcrypt/scrypt/whatevercrypt can be relatively easy parallelized.
My idea is to make each processing unit requiring a lot of RAM for intermediate data thus adding "GByte" dimension to "GHash" race and then keeping GByte-GHash profile similar to that one of the general-purpose PCs somehow.