Saw this site few days ago on HN:
http://bugmuncher.com/
Seems to be doing same sort of thing, except that the processing is being done on server side rather on client.
BugMuncher founder/developer here - BugMuncher takes the entire DOM tree from the client and recreates it for the screenshot, so as long as any external CSS/images aren't behind authentication, it works fine on authenticated pages. In fact, most of our users use BugMuncher specifically on auth protected areas.