The price will go up.

Right now there are a bunch of professional miners who are attempting to make a living from mining bitcoins. Every month (when their bills come due) they sell a bunch of coins and drive the price down. When the difficulty vs. payout doubles, their income will halve and many will shutdown because they can't cover their costs anymore. You'll also see a flood of used mining rigs and GPUs hit ebay at that time.
The difficulty will gradually trickle down until only the most profitable miners and the enthusiasts are still mining.