WEll the potential for mining difficulty in teh future can drive the price up now. Good example is ASICs that will be coming online in the near future....it is called PRICE ANTICIPATION of an event.
How does knowing the difficulty is going to go up influence the price? There will still be the same number of coins rewarded every ten minutes.
If people knew a gold rush was coming because better technology was on its way...people would be buying up the supply of gold in advance....especially if this new technology being used implied that mining for gold would become that much more difficult.
Get it now? Simple logic.
Edit: Increased demand can translate to increased mining hash power = harder to mine = more power = more equipment = more cost.
But I think things are actually going the other way: ASICs make mining easier (for those who have them). If it takes less effort to get the bitcoins, since the miner is more efficient, then that miner will be willing to sell at a lower price. To use your analogy, if you know a gold rush is coming (and the supply of gold will go up), then why would you buy now? You would offer a lower price in anticipation of the increased supply.