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.
You mean you're not one of those Folding @ Home guys, whose idea of fun is to spend their extra money on hardware to crunch numbers?

I'm not such a (what's the most derogatory word for geek or nerd?) that I'd spend even $20 on electricity for some silly distributed computing project.
I'd rather take my wife out to dinner or ANYTHING I can actually enjoy. I guess I have a life...
your wife was in the room with you when you typed that right? hahahhaha
Nah -- taking my wife out to dinner involves A) quiet time with my wife away from the kids, B) good food, and C) improved mood for both my wife AND myself.
So what's not to like about that?
I just don't see the benefit of spending electricity on something of dubious value. Scientists have access to all sorts of computers, including clusters of off-the-shelf PCs connected by ETHERNET, as well as actual supercomputers. They don't need my $20 in wasted electricity per month. Besides, I'm firmly convinced that cancer has been cured several times over. They just won't let us have the cure, since chemotherapy is so profitable. And let's not forget that the Georgia Guidestones call for a world population of only 500 million people -- curing a killer like cancer would be counter-productive to that goal.
Long story short -- I'm not a sucker. They don't need my computer, or my electricity.