Good question. If we assume efficiency of a custom ASIC is 10x that of an FPGA, then mining efficiency (bitcoins mined per amount of energy spent) for a custom ASIC will be exactly the same as that of an FPGA, if difficulty rises by a factor of 10. This, of course, won't happen all at once, but it might be the reason BFL is selling these units so cheaply: they've done the math, and the math ain't promising (if you're a miner). For BFL it's a goldbitcoin mine.
Maybe they get the ASICs in September (and the devices done) and let them rum for 4 weeks till october for 'testing'. But you are right, in the long run, they will make more money selling mining hardware.
