Totally coincidentally, I was just working on a User Script that does this for all Hacker News posts; basically, puts the full, unqualified hostname in the parenthesis after the post. It's surprisingly simple.

https://gist.github.com/1522657

(UserScripts.org is failing me. To install, simply go to that gist and click "Raw" on Chrome, or on Firefox with Greasemonkey installed).

EDIT: I've made some changes since initially posting, including removing the www. when it appears, and not making unnecessary DOM alterations if the domain wouldn't be changing. Also did a little bit of readability clean-up.