PW Forum / Generic backend to allow better cross-domain mashups

I'm working on a system to allow cross-domain mashups.  The basic idea is to allow you to consume my service using the <script> tag, but to pass the address of an XML, JSON or HTML page.  I've documented it here:
http://davesxss.blogspot.com/

I'd be interested in hearing feedback.  Is the security situation terrible?  What about speed, usefulness, etc?

Thanks!
Dave