Smart Card Authentication for Citrix Web Interface is a feature allowing users to authenticate using a physical card holding a certificate issued to the user.  This is a pretty cool thing, but it requires that the Web Interface server be a domain member. Nobody wants to expose a domain member on the DMZ of their network, so typically Smart Card Authentication is not available from outside your network. 

Jay Tomlin has posted a nifty solution to this problem here: http://www.jaytomlin.com/blog/2006/09/web_interfac...c.html