If hypothetically bitcoin fell to <$5 and stayed CONTINUALLY under $5 for 30 days (long enough for miners to get next electric bill) you likely would see some behavior change.
That's the key right there. Most people will stick it out at least until the next power bill hits and maybe even for two bills, hoping to see a turnaround in price or drop in difficulty that will keep mining profitable. There's definitely some people out there that will continue to mine regardless, but I bet some of the power goes dark after a while. For example, I'm running 11 GH or so and it's costing about $500/month with a great .085 cents/kwh power rate. If it gets unprofitable for me I'll still probably keep a GH or two running to help the network, but I can't afford to throw $400-$500 at it each month with no return for very long.
People need to factor in that winter is coming in the Northern Hemisphere as well. Both European and US miners will be turning back on as there is no other spaceheaters with a return, except the aforementioned marijuana farm. I used to dump my grow exhaust into the house in winter, to my ladyfriend's chagrin (she doesn't partake). It kept things warm at no additional cost, but you don't run odor control like ozone (toxic) while you are blowing into your living space so the house tended to get a bit fragrant.
I know here in Los Angeles (on LADWP at least), electricity rates also drop beginning in September, almost halving my cost per kwhr. Thank god for public utilities. My friend on Edison (the private one) pay almost triple my rate and are only 15 miles away...