Such a move is clearly justified by the fact that while Euros are involved in nearly 40% of all trades, the BIS data shows that only 11.5% of the trading actually occurs within the 17 Eurozone countries (see the table). It only seems reasonable that the Eurozone should have some control over this speculation.
So, imagine that all governments agreed to impose the tax on currency exchanges, and that the revenue generated would be paid directly to the central banks for each currency - the Federal Reserve in the USA, the Bank of England in the UK, the European Central Bank for the Eurozone and so forth. As you can see from the table on the right, this would mean that each currencies central bank would receive an amount that depended simply on the degree to which their currency was the victim of speculation. The US Treasury would be the biggest winner with 42.4% of the revenue, followed by the European Central Bank with 19.5%, the Japanese Central Bank with 9.5% and the Bank of England with 6.4%. But all countries would get a share of the pie. And when several countries share the same currency (as in the case of the Euro), then revenue can simply be shared on the basis of population size.
How much could they raise this way? Well, obviously, it depends on the rate of the tax. My suggestion would be to fix the rate at the average rate charged by the banks and the credit card companies for making transactions in foreign currencies. In this way, you could kill two birds with one stone because the banks would be under very strong pressure to reduce the extortionate and totally unjustifiable 2-3% charges they currently levy (see this site for a list of International Transaction Charges for UK based Cards - of the nearly 300 cards on offer, the majority charge either 2.95% or 2.99% and a third of them charge 2.75%. There are two that charge 2.50%, two that charge 2%, and amazingly, there are actually 12 that don't charge anything. You see, it can be done!).
Obviously, if all cards decided to drop their International Financial Transaction Tax charges, then the central banks might have to impose their own number for the financial transaction tax. I would think that 0.1% would be fine, and could generate up to $1 trillion a year if the traders keep speculating like they do currently.
The other neat aspect of this method is that it directly punishes the markets when they attempt to attack a currency. The more they speculate, the more they have to pay. It seems extremely fair and reasonable to me.
Is there anyone out there listening?