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.
Is there any information out there as to browser support? Has any materialized at all? It seems like a great technology, though.
No implementations yet, unfortunately. I wouldn’t expect to see any significant implementation effort for about 2 or 3 years.