[cotc2001]

Not sure if this would be of any interest to you, but how about tying it in with maxmind geoip so that initially when a visitor comes to your site it will display the currency in their local currency without any user intervention, thats what we have done with our site (including displaying if the price should be displayed inc VAT or ex VAT) and it works very well (when customer creates account it overrides it of course just in case there is a mistake)