All I can come up with is scarcity. How often is is used seems to be a long way from being the limiting factor. Lets say that Overstock and all other recently announced BTC accepting companies take 1000 BTC per month revenue, no where near the amount of BTC that are actively traded. The people using BTC in transactions will just go and rebuy them with Fiat. The companies won't be stockpiling them, they will be exchanging for fiat straight away. So it doesn't make much difference if the price is $100 or $10000, Bitcoin can be used just as easily either way.
It becomes a limiting factor when the turnover of Bitcoins is higher than the value of the bitcoins being actively traded. Then the price would have to rise. Then I would say that the price is based on fundamentals, whereas now it is based on speculation. Gold based on fundamentals should be a few bucks an ounce, so I'm not predicting a crash, just saying that good news won't necessarily increase the value of bitcoin (in my opinion).
His words, condensed: bitcoin is very very over-speculated