O'Reilly Network: Mozilla as a Development Platform: An Interview with Axel Hecht:

"Writing an XULRunner application is as easy as writing a few text files, there is no compilation involved and for the most part, you don't have to ship any binary code besides XULRunner itself. Applications can readily use the localization support, theming, and even the extension manager and its back end. We already provide XULRunner nightlies, so people can test it."

And we know from those recent job postings from Yahoo show that at least one big company is doing something with XUL.