The price has gone up by 20% in 24 hours, not just doubled in the last 12 months - I think in twelve months (according to bitcoin charts) it has gone from around $10 to around $600 - 4900% increase. That's clearly unsustainable and yes it does look like a speculative bubble, particularly when people expect it to continue to grow indefinitely in price without a commensurate rise in usage as a currency.

It'll be interesting to see this play out as it's a great example of a completely unregulated market at work.

http://bitcoincharts.com/charts/mtgoxUSD#rg360ztgSzm1g10zm2g...

I don't see bitcoin becoming a world currency without some large changes to the protocol - for example to track users and link them with real world IDs, reversible transactions etc, and there are also political reasons which would make it very difficult - governments enjoy being able to fund deficit spending and control the economy by manipulating the currency, they won't easily give up that power.