I suspect that the price of a Bitcoin is pretty much separate from the block rewards. The difficulty is only related to it in the way that if mining becomes unprofitable due to the Bitcoin vs. $ Price vs. power costs, smart folks will stop mining and there is less processing power in the network and the difficulty will go down. But my experience is that an awful lot of folks aren't all that smart and will continue to mine even when it's not profitable. You also have the folks that will have to continue mining for a said period because the have a contract anyway (colocation, racks, office, etc.).

Currently a very limited amount of BTC trading can influence the worth of the BTC drastically. A week ago, the trading volume of 65.000 Bitcoins over a period of two days resulted in virtually cutting the 'worth' of the BTC in half (on mtgox). Certain 'news' can really gut the worth of the BTC (such as a hack on robbery of an exchange). So I think that the worth of the BTC actually drives the amount of miners and thus the difficulty, then the other way around...