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.