As Mossop mentioned, I've been working for the past week on getting XULRunner nightly builds up and running.
I'm happy to announce that they're now available!
The first builds of XULRunner for Linux (i686 and x86_64), Windows and OS X (i386 and ppc) for both mozilla-1.9.1 and mozilla-central (1.9.2) are finishing up, and are available (or will be soon!) at http://ftp.mozilla.org/pub/mozilla.org/xulrunner/nightly/. Fresh builds will be available every night.