as the reward gets lower less people will mine! The demand for bitcoin should also go up. Less people making bitcoins then value of bitcoin goes up. When the difficulty shoots up most gpu miners will quit.
Ah, that may not totally be the case. Demand for Bitcoin is still tied mostly to speculators, and it's not likely ever single person GPU mining that drops out will buy BTC to replace their mining. They may just flat out stop dealing with BTC and sell or hold all they have. Throw in the fact that ASIC miners are going to be dumping BTC like mad trying to recoup their costs, there's going to be a lot more supply than demand for a while.