Pretty cool! Can't be more simple. I don't know much about website programming but isn't it possible for website's to sometimes see where you were just browsing and thereby get the address?
When you click a link on a site, most browsers send the url of the page you were just visiting to the server of site you clicked on. So, this would be an issue if instantwallet.com had links to other websites.
Very nice site. I will recommend this to anyone I introduce to bitcoins.