I implemented a little prototype of this. This is very doable. The beauty is that it didn't use HTTP-Auth, which meant that a user can be using Google Reader or

I implemented a little prototype of this. This is very doable. The beauty is that it didn't use HTTP-Auth, which meant that a user can be using Google Reader or another third-party browser, and still get access to the paid content only if they authenticated correctly, but this still happens from within the feed reader, without clicking back to the website.