http://jsfiddle.net/gS4PL/1/
It goes out and gets the current exchange rate then you fill in the USD amount and updates the form with the EUR amount.
Feel free to use this if you need it, the xqlAjax class I got from:http://fragged.org/cross-domain-ajax-calls-via-yql-as-proxy-and-mootools-jsonp_1028.html
