It is true that price determines difficulty, not the other way around.
I still disagree with that (if I have a cost to overcome, I won't sell a BTC under cost, thus however long and costly the BTC was to produce, this is how high I need to let price go up before selling).
That doesn't make sense. Miners hoarding BTC would have such a small effect on the market comparatively. If it suddenly costs $1000 a day to mine 5 BTC on average, that doesn't make BTC rise to $200 each. Miners would WISH it would rise to that, but effectively no one will continue to mine because it is too costly, and they will only re-enter mining at a level that makes costly sense IE the difficulty will fall to match the appropriate price of 1BTC in USD.
The only way difficulty could affect price is if miners could mine at whatever rate - but we know that the bitcoin system adjusts to only allow 50BTC per 10 minutes to enter the supply. Since miners DO NOT AFFECT SUPPLY (really, it doesn't matter if one dude is CPU mining at low difficulty or 1million miners are mining at an insanely high difficulty, the same supply is made!) they cannot affect price.
However, price affects how many people choose to enter (or exit) the mining, thus changing the difficulty.