I've spent the last little while working on an XBL editor which I've called Xabyl, which hopefully avoids any naming issues Phoenix and Chimera have been having.
I haven't decided where to put it yet. I may stick it on mozdev.org or I may suggest adding it as part of XUL Maker The latter might be good, but XBL can be used outside of XUL as well.
Anyway, I've put it up on XULPlanet for now so people can play with it.
Next task: more XUL examples.