Since the XBL seems to be a good moniker, leaving it in the title for now, but Hixie says it'll be just part of HTML spec.
The draft (work in progress) is here: http://dev.w3.org/2006/xbl2/
The use cases are here: http://wiki.whatwg.org/wiki/Component_Model_Use_Cases
Retitling to better reflect present day. Here's a good intro for Web Components: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
We're tracking the new implementation effort differently.