XUL has been around for a while now in the form of Mozilla, Firefox, Thunderbird, and of course, all associated extensions. The engine which powers it (Netscape Gecko Layout engine) is very mature now - feature rich, and stable. It is a runtime with no equal - because it was built to power a web browser, it is the most webcentric runtime around. There have been few other notable stand-alone XUL apps due complexity. The complexity of setting up a new XUL app is a departure from the simplicity of coding XUL interfaces. This issue is now being redressed by the XulRunner project
which aims to make building your XUL app easy.
I'm excited about this.
This is a big deal.
If you're a web developer, you should considder becoming an early adopter of this technology. I'm rolling my sleeves up and getting my hands dirty on a rather big and complex project that I hope to open-source at some stage. Hopefully, in my travels, I will learn enough to put together a basic tutoria here.
Wish me luck, and stay tuned