I realised that Blueprint has very little in the way of documentation to help people figure it out. Despite this, usage of Blueprint seems to be taking off, with nearly 700 specifications in the system and quite a lot of Launchpad karma getting dished out daily. But documentation will help, especially since Blueprint is starting to take shape and will be at the heart of our planning for Dapper+1.
So without further ado, I give you:
Official Blueprint Docs (v0.1)
Enjoy, and comments, updates and clarifications welcome! Note, some of those features discussed have landed in the code but not yet moved to production, so you can find them at http://staging.ubuntu.com/ rather than http://launchpad.net/ for the moment.