One could argue that a price increase because of a difficultly increase already got priced in from it hitting $10 and change until now. In the new york stock exchange, stock prices typically move based on estimations and expectations of future events. Since the difficulty change is known in advance, those that believe that will cause the price to go up would buy in as soon as they know of the difficulty increase. Why would one wait until the difficulty increase actually happens to buy in if that is your reason for buying (more)?
This is true but this is a brand new market and as we saw over the weekend price can fluctuate rapidly. I'm in the boat that we'll see a small increase (maybe $25) but nothing too spectacular.