A run to what, whom, where?? People know that bitoin is not redeemable to anything. Personnaly if bitcoin price was to decrease dramatically, I'd buy everything, just as I'd buy several kilograms of gold if the price went to 35$ an once. I doubt I'd be the only one.
The US dollar (which isn't redeemable for anything either), imagine suddenly everyone sold all their BTC on Mtgox and then more people and more people. You can have crash in the bitcoin market just like you can in the stockmarket.