Just a quick note for those of you not part of the MT. My plan is to test-drive the single sign-on on a bit of the site that you non-MT folks don't know about - it's a much used part of the site for the MT though.
While making it understand an auth pass-through from the forum will be nice for us in the MT, my actual thinking here is that said bit of the site uses it's own authentication mechanisms any way (a separate bit of the database, it's own username/password handling code) and I can play with it without overly hurting anything that would affect everyone. So, don't expect things to change very much for a wee while yet...