I saw too many questions about bitcoin losing value.
But in my view, 21 million bitcoins is relatively less money (ie., units) than other currencies. Why wouldn't Bitcoin's value rise simply due to supply/demand when it is used more widely?
Because there is no guarantee that bitcoin will be in demand in the future. Here's an example, in a recent Gawker article they put "(RIP)" when naming Bitcoin .. as if it had died. http://www.gizmodo.com.au/2011/09/is-digital-money-the-new-way-to-buy-drugs
Of course, the existing $35 million or so valuation of the currency come from those speculating that the value of the currency will at some point in the future buy more than it will today.
Whether or not this will be a stratospheric rise nobody knows but it isn't unprecedented, just not typical nor expected by most. But there are those, such as Rick Falvinge, who believe it. Rick describes his thoughts here:
The value will definitely rise if Bitcoins are ever used widely. But there's no guarantee that will ever happen. Even if you think crytpo-currencies are awesome and that human beings will be using something like them for centuries, it' still entirely possible that it will not be Bitcoins but some similar crypto-currency.
It is likely that almost all of the current value of Bitcoins comes from speculation. In fact, the recent drop from around $11 to around $5 is likely due to people thinking the chances of Bitcoins becoming mainstream are lower.
The point is, widespread adoption of Bitcoin in the future is very far from a sure thing -- even if you think widespread adoption of crypto-currencies is a sure thing.