Is there one?
I can find articles that talk about it's implementation from 2008:
But I can't seem to find any actual documentation on how to use it. Anyone used it? Does it exist?
Good Question!
Unfortunately MS stopped investing into OpenID for now and there are no plans to be found when they will reopen their efforts for the MS Live ID to become an OpenID.
Official word is in this Microsoft Privacy and Safety Group Blog Post from August 2009.
"Best" tidbits:
...we are in the process of closing the OpenID CTP endpoints...
...we do not have a schedule that I can publicly share for when we will release full Production support of OpenID for Windows Live ID users...