Yo
I'ma let you finish...
http://www.polyvore.com/cgi/img-thing?.out=jpg&size=l&tid=11793371

but what I would consider doing is setting USD as default. Pricing things in terms of USD, updating just the BTC currency rate table at cron.  Then in your template header, set the $_SESSION['currency'] to bitcoin so it is the default.  This way you don't have to continually modify your script every time you add a new product.