I've just shut down my (deepbit) miner for another day, as 'free' power from my solar PV array has finished for today. I wouldn't discount the possibility that a large proportion of miners on deepbit have 'free' power (solar PV, or stealing it from someone else), are on fixed power contracts (which will go up long after they finish mining due to the lag effect), or have some of the cheapest power on Earth and don't mind having hundreds of dollars in hardware producing cents per day.
While that might be true, it isnt driving hash rate unless ALL of them have free electricity or dont care. The miners that do have free electricity (or dont care about the price and act irrationally) will drive out the miners that dont have it and hash rate will still plummet.
What you are seeing is more likely a delayed response. Price plummets, miners reevaluate but dont instantly pull the plug or sell their gear. It takes time to convince them the price is going to stay that low, it takes time for electricity bills to arrive, it takes time to find buyers for their equipment. Hash rate will lag bitcoin price, but in the long run, will always correlate as long as there is a single rational miner left that actually pays for his electricity.