Interesting though. I wonder how low the exchange rate has to go before ASIC stop producing more than operating costs, and miners start shutting off their rigs for a bit?
That's an interesting question -- no idea what the answer might be now, but you can get a first glimpse from an historical perspective by comparing the graph of total network hashing rate with the graph of USD vs. BTC:
http://bitcoin.sipa.behttp://bitcoincharts.com/charts/mtgoxUSDThe latter has a checkbox for "Custom Time", which lets you set whatever interval you'd like.
When BTC took its sudden dip in mid-2011, for example, there was a less sudden drop in total network power which began soon after. No doubt there's more to the decision, but the exchange rate is bound to be a big part of it.