I am YAXULNPC. Yet Another XUL Newbie Playing Catch-up. Oh oh...
XUL XUL XUL. XUL is Cool. there.is.only.xul
So, like seemingly everybody right now, I'm in learn-mode. There are a LOT of good resources already, which I think is a feather in the cap of the open-source folks, and of course the Mozilla folks specifically. Here's a Google Search for XUL.
Neil Deakin: XUL Tutorial Appears to be a must read... still working my way through it
My Sidebar Developer's Guide (Beta Draft)
XML.COM: Fooling with XUL