How about this for some perspective. The current bitcoin economy is somewhere between Micronesia's and Tonga's total GDP (
http://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nominal)). We're not even talking about a tiny part of the global economy, we're talking about a tiny part of a tiny country's economy.
I'm gonna rant a little, kind of share what my imagination has been conjuring lately...
I tend to believe that the network effect of bitcoin is it's strongest and most vulnerable point. The more people use it, the more people use it - and the less people use it, the less people use it. On the one hand, if it doesn't stick well with people, it will have a very very hard time gaining traction. On the other hand, if it does start catching on significantly, it won't stop. That's why I think it will follow a classical exponential adoption rate.
I'm not going to say that I expect bitcoin to take over the world economy, but since there is a [variable] hard limit to the size of the bitcoin economy at full saturation (theoretically the global economy), we will see a very clear "log phase" in a market cap graph as seen with, e.g. bacterial growth (
http://en.wikipedia.org/wiki/Bacterial_growth). When the bitcoin economy reaches full saturation, everything from 2009 until today will look like a straight line, because the market cap will be many, many orders of magnitude larger than today.
Edit: I will add that having more than one medium of trade is not fun. If everyone is buying and selling in bitcoin, you are more likely to sell and buy in bitcoin respectively.I believe bitcoin's most interesting phase is most definitely not this one. I believe bitcoin will catch on in exponential fashion. That is to say, the jump from the Marshal Islands to Micronesia (roughly four weeks) will take around the same time as the jump from the UK to China. I kind of imagine adoption rate to be somewhat similar to Facebook's, except bitcoin might have a sharper exponential rate.
http://bylimedesign.com/facebook_histodical_users.pngThe downside is that the transition will be harsh. And I mean dire. Governments will not take this lightly. Taxation is a bitch with bitcoin. Central banks won't either. Bitcoin is not fiat. And since both are tied to each other for survival the way things are currently run around the world, and considering both will probably use whatever means necessary to avoid competing with bitcoin directly, I believe violent means will not be passed over... Bitcoin will be fought in similar tactics as p2p file sharing was/is fought by the record label industry. Idiotically. However, as dinosaur as fiat may be in the face of bitcoin, this war will be orders of magnitude larger than that of the record label industry. And the means will be orders of magnitude more fierce.
That "log phase" will be the defining moment. That's when the public will decide if bitcoin is here to stay. If you think today the bitcoin is volatile, imagine the events that will influence bitcoin when Vietnam/Afghanistan/Iraq-scale war will be waged to keep business as usual in western civilization? On the other hand, how do you take down a Facebook with out any central servers or operators?