PyQt is more of a GUI library, so you'll need to use something else for XML. It seems like Expat would be a good choice. Have a look at the documentation here:
http://docs.python.org/library/pyexpat.html
If I want to see @mentions in my web browser I'll just open up a tab as I normally do. I don't need this to open it. The nice thing about building a standalone Twitter app is you can create your own unique user experience.
Anyway, I was playing around with this a bit more and was able to turn this into standalone Mac OS X app with py2app. It worked pretty well.