Over at Hyatt's Safari site, he has links to portions of a cross-blog conversation about XUL, XAML, and GUI markup languages in general. Check it out if those things are up your alley.
I find the topic of browser driven applications interesting. The ability to build an interface that is lightweight, easily customizable, and made up of such easily understood building blocks is very empowering. I've always enjoyed web-development because of the quickness with which one can build something and deploy it. Although more powerful (and probably because of this) languages like C++ are much more difficult to use and take much longer to create a working GUI based app. The ability to use free tools like text editors and my browser to deploy an internet aware and distributed application is great in my opinion.