You guys who keep peddling this bullshit theory are disregarding two things:

a) No one who sees fit to "invest" in Bitcoins from an outside perspective really gives a shit about the cost to mine. If they do any sort of research, they might look at the overall hash rate of the network and compare how much it would cost an outside party to attack the network, but I doubt most even do that. They certainly don't give a shit about what kind of ROI you expect on your mining rig;

b) There are always going to be people who mine cheaper than you do. You're thinking that Bitcoins cost $X to mine because your rig cost this much and you're expecting to pay it off over a 6 month period and the difficulty went up so blah blah blah. No one gives a shit. Johnny CollegeStudent just bought a 2x 6870 rig with his grant for college and his dorm room's power is "free" (in his eyes). He doesn't give a shit that you really need a $15+ Bitcoin this week for your ROI plan not to nosedive - he wants a dimebag and he doesn't have a Silk Road account so he's going to sell it for whatever he can get on MtGox.

Your argument might hold water if all miners held the line and no one wanted to drop below a certain figure - you're a fool if you think that's gonna happen. My 5770 is all paid off, so depending on what the market's doing I'm going to mine for whatever I feel comfortable making a profit at (and stop beating the shit out of my brand new, free GPU if it falls below that), and I don't particularly give a shit to hold some line or another - I'm going to take $16.80 today than risk having to take $13 tomorrow, and I'm sure I'm not the only one.