More than anything else this is probably determined by electricity cost varying in different areas.
In my area we pay $0.10/kwh but anyone paying above approx. $0.17 has either shutdown or is thinking hard about it now. The variance of cost is wide - from "free" in shared dorms, to 0.01 in Venezuela to 0.40 in some European countries. Even the tech haven of California has pretty nasty electric rates.
I wouldn't be surprised if someone put together a map of miners from over time that we'd see whole areas go dark over time like a grid shutting down. As the price drops miners are forced to confront whether they want to spend electricity to hold bitcoins for future possible price increases, or just go dark.
And in sync with this the price of 5830 gpus on eBay has dropped from rough $135 a couple months back to more typically $100 or less now. This is all a natural outcome of people getting into mining to make money rather than for philosophical reasons, though there is always some blurring of the boundaries.
Keep in mind that as difficulty falls this improves the return and it should slow the hash rate decline. So everyone is watching that too and deciding what they consider the break even point to be.