You'll know when they ship because difficulty will sharply increase. Then everyone who wants to mine will need one to compete. Or they dont ship and things continue as they do today.
Difficulty will rise anyway. GPUs and FPGAs are easily profitable at these exchange rates. ASICMiner has 50Th of chips on hand to deploy this month and just ordered 200Th more. Avalon has all but completed shipping of batch 1 and batch 2 shipping is scheduled to begin in less than two weeks.
BFL is the slow pony in the ASIC mining arms race.