On second thought, I've read somewhere that even IF diff hits 16 Billion, these Antminer S1 units will still be profitable, depending on your electricity rate, of course.
It also depends on exchange rate. But at the current exchange rate, Ants with $.12/kwh power are profitable up to about 50 B difficulty! I think the exchange rate is going to go up this year, and a lot of people have power cheaper than $.12, so a lot of S1 Ants will be good to 100 B or more.