I don't see why you'd junk a B1. The cost is already sunk and there is nothing but upside. If they can ship multiple skus, up to, and including a higher clocked 512sp part, they will be able to charge a premium.
The chance of AMD coming with a faster part in the near future is there. I don't think NV would like to see AMD release a refresh before the GTX480 actually sells and they'll have to play second fiddle for another six months.