I can't comment on ars's answer, but I actually got Roland Tapken's code running using QtWebkit and it works quite well.
Just wanted to confirm that what Roland posts on his blog works great on Ubuntu. Our production version ended up not using any of what he wrote but we are using the PyQt/QtWebKit bindings with much success.
Note: The URL used to be: http://www.blogs.uni-osnabrueck.de/rotapken/2008/12/03/create-screenshots-of-a-web-page-using-python-and-qtwebkit/ I've updated it with a working copy.