Hey Andy,
We have good and bad news.
First with the good news: As of March 2013, Authorize.net now supports Canadian Dollars (CAD), Pounds Sterling (GBP), and Euro's (EUR). More info here.
Bad news: We have not implemented this change into our existing Authorize.Net payment method.
I just created a ticket requesting this implementation, however, we do not know if and when it will be implemented.
-Kalvin