Looks cool, but you should make it a greasemonkey script, so I don't have to press it on every page.
Also, give the bookmarklet link on the page a better name than hn.js - to make a bookmarklet, the easiest is to drag that link to your toolbar, but that gives it a title of hn.js.
I like that its a bookmarklet and not a greasemonkey script: that means I can use it as well in Safari.
Yes, there is CreamMonkey. However, CreamMonkey is an inputmanager. Which means: A. it doesn't work in 64bit Safari and B. gets loaded into every (32bit) app on the system. That's something that I don't want to deal with.
Looks cool...the font is a bit hard to read though. Perhaps you could add a bit of code in that checks the header link styles and uses those same background and link colors for the little popup.
Also, give the bookmarklet link on the page a better name than hn.js - to make a bookmarklet, the easiest is to drag that link to your toolbar, but that gives it a title of hn.js.