The first public working draft of the XBL Primer has just been published! It is being edited by myself and Marcos Caceres.
For those of you who were wondering when exactly I was going to complete my series on XBL (see part 1 and part 2), that content has been integrated into the primer. If you have any comments about it, feel free to post them to public-appformats@w3.org (archive), leave a comment here or mail me directly.