Despite quite a bit of interest when XBL 2.0 was developed it has not made it into browsers thus far. Perhaps because Jonas Sicking is too occupied, or perhaps it simply is too complex. The drive for XBL started moving again late last year when Ian Hickson simplified the specification. Instead of a new language XBL would become a set of extensions to HTML.
In parallel, Dimitry Glazkov started drafting Component Model Use Cases, a WHATWG Wiki document anyone can contribute to. Effectively taking a fresh look at what we want nowadays from XBL. He also wrote a great post for anyone not familiar with the types of scenarios XBL will tackle: What the Heck is Shadow DOM? It is a complex subject, but the way he conveys it makes it look easy.
Now I have been hopeful for XBL happening pretty much since we started working on HTML5 in 2004 so I am not too excited yet. But for the first time in almost four years we are making progress again.