Example so far can be found here.. signup at blizzforums.com and then watch what happens when you try to log in, you're user from blizzforums lets you login to the other site
Currently the MS passport style system is disabled until we figure out some stuff with proxies.. shouldn't be too long though.
http://cms.blizzforums.com/
Oh, if you can't access that url.
For windows 9x users,
Edit "C:\windows\hosts" add the following.
For windows NT/2000/XP users edit C:\WINNT\system32\drivers\etc\hosts and add the same code portion.