A single global currency is inevitable and the ongoing global financial crisis has precipitated another small step in this direction when all of the major central banks around the world announced a coordinated financial easing policy. Just like it took a world war to create the League of Nations and second world war to create the UN, this financial crisis will help create a financial UN to displace the IMF (which in this analogy is the similarly ineffective and ill-fated League of Nations).
As an aside, I don't think these rate cuts will achieve anything useful re: the current crisis. The fundamental problem is excess debt and the solutions so far have been attempts at making debt easier to acquire. How will encouraging people to take on more debt solve the problem of too much debt?
But that is unimportant. What is important is that it is becoming obvious to everyone that we live in a single tightly interwoven international economy that cannot be controlled by individual national institutions. When this crisis eventually passes, there will be efforts to create something like a UN for the central banking system, though hopefully the design of this institution will take some of the lessons from the successes and failures of both the UN and the European Central Bank.
In the long run, as this institution is seen to be effective, there will be increased pressure to move to a single currency, by adopting the dollar, the euro, or the yuan or creating a new currency.
All of these actions will be supported by the markets because they will reduce risk. Lower risk means greater long term returns on investment.
So, that is the silver lining for the financial crisis, if you were looking for one.