Show me one shipping unit that comes anywhere even CLOSE to BFL's offering. Go on... send me a link.
There are at least 3 FPGA miners shipping today, with ~2X the MH/W of BFL:
BitcoinFPGAIf BFL takes off and starts driving difficulty - no worries - all of these units will still be profitable.
If these Spartan6 designs begin to drive the difficulty - they will make the BFL units unprofitable.
It doesn't matter if it was cheaper per MH up front, once it's losing money every month.
-rph