After the 12 original countries (The Netherlands, Belgium, Luxembourg, France, Spain, Portugal, Italy, Germany, Austria, Greece, Finland and Ireland) changing their currency in a massive operation a few years ago, last year Slovenia joined. In January, again 2 smaller states, Malta and Cyprus, will start using the Euro. 320 mln out of the 495 mln inhabitants of the European Union use Euros. Slovakia will probably convert January 2009.
The Eurozone states need to adhere to some pretty strict rules regarding inflation, trade deficit, etcetera, so next to the emotional issue and the release of sovereign interest control, it is not simply a trade of one coin to the other.