To make Windows Parental Controls work better with browsers and other programs that support http, the filter in vista sends back a custom error number when the page is blocked.  This number is 450, an app can recognise this error code and display a blocked message or allow for overrides to be done based on the url that was blocked.  This is how IE recognises blocked content and shows a gold bar to allow the user to override the urls blocked on the page.  Firefox has plans to support this as well.  WPC will recognise the various versions of http and return a header that is correct for each different version, from 0.9 upwards.