There was some discussion of bambookit on one of the Mozilla lists and I
think there may have been some question of how closely it followed XUL. I
have never used it, and the applet building block might be problematic if
you are aiming for a rich user interface that serves a wide audience. I
don't know the other competitors for XForms-like environments right now
either, but one thing cforms has going for it is the "continuation"
support in Cocoon, it nicely gets around all sorts of state-handling
issues you often run into with complex web applications. I have heard of
one case where a web application tests for mozilla and uses stylesheets to
produce XUL on the fly. If XAML, or whatever Microsoft's technology ends
up being called, is somewhat close to XUL, server-side stylesheets might
be one way to mediate a rich interface between browsers.