I wonder whether the sub-dividability of Bitcoin is one of the factors allowing such a sustained growth. Traditional commodities aren't sub-dividable like this stuff is. Each bitcoin is divisible into 100 million parts, known as a Satoshi, so effectively, the price could keep rising, yet new investors won't be completely priced out of buying bits and pieces until the Satoshi itself becomes the limit.
That could be very different to e.g. a property market. You can't really sell half a house, you need to find someone willing to buy the entire house, and when the speculator money dries up (e.g. no one investor can afford the whole house) then the market prices plateau and then fall as people start selling to invest in something else. Company shares are the same: you can only buy them in integer amounts. If houses and/or shares were sub-divisible, then with a price rise, you never know what difference that would make. if you're always able to buy e.g. $1000 worth of house then of course, property bubbles would most likely continue on far longer than they actually do.
So, with bitcoin, no matter how much the price, it's always possible to e.g. buy "$300 worth" or whatever other amount is acceptable for people willing to take a punt on the market. Theoretically, the price could keep rising, until the point at which the Satoshi itself becomes the bottleneck to buying or selling reasonable amounts.
However, what's most likely to happen is that after some time, a clearly superior crypto-currency is developed, at which point people start to mass-migrate over to that blockchain leaving the original bitcoin blockchain basically as a deserted backwater, full of wallets with lost keys, and diehard geek fans tinkering around.