Ian Hickson has some interesting quotes from a recent XUL naming debate between him and, if you've been following XUL, you should know who else.

We stopped working on the XUL spec itself when XUL Planet started documenting XUL in more detail than we had time to do ourselves.

and:

Most of the current XUL work is being done within standards organisations with strict NDA policies. If you are a ISO, W3C, ECMA, or IETF member let me know and I can show you the relevant links... I'm under NDA from one of the above groups, in which XUL is being discussed.

and even:

XUL _is_ in active development, we are currently taking feedback into account and are more carefully defining the XUL box model.

All quotes from here.