I did some calculations a while ago (see http://bitcointalk.org/index.php?topic=1918
) and, using updated values, I would estimate that the expected number of blocks you can generate (ever) with one of these machines, given current bitcoin network growth, is about 11 (=550 BTC). So it should more-or-less pay for itself, and if bitcoins continue to appreciate against the dollar, then it will more than pay for itself (NOT counting electricity costs). Hah, I was expecting the opposite conclusion. I should buy one of these.
Expected number of blocks: E=Ch/(a-1)/D
H_m = hash at min difficulty = 0x00000000FFFF00000....
H_M=max hash value=0xFFFFF....
a=difficulty growth = 1.153 (approximate, at time of writing)
EDIT: this is interesting, if these miners will pay for themselves, why doesn't atlas just keep them all for himself and use them to mine? Ok, the answer is obvious, bitcoins don't buy bread yet (though someone did buy pizza).