It's no longer profitable to invest in new mining rigs, moreover, many people are scarcely making any profit with such low exchange rates.
There's a little something called sunk cost, and it's by no means a fallacy: when you've already committed a large budget of money and time on a mining project, you can't simply pull out if you are not "living the dream". It makes sense to keep mining for as long as you can pay the power bill, any extra is passive income that goes towards amortizing your sunk cost.
A 2Mhash/Joule, 2Ghash/second rig draws 1KW (24KWh/day) and earns ฿1.15/day at current difficulty.There are places (Canada) where you can get a KWh for under 5c, so those people will still be mining at 1.2$/BTC, even if difficulty does not change. Conversely, people who have quality hardware but expensive electricity will (20c/KWh) should pull the plug below 4.8$/BTC.
On the other hand disruptive technology could arrive any day now (ASICs), and easily increase the difficulty by an order of magnitude with no price impact.
