There have been some great efforts to discuss and specify what a single-login implementatoin will look like in recent months. However, the major disagreements form the last bout of specifications -- how to deal with name conflicts -- have not yet been addressed.
See http://meta.wikimedia.org/wiki/Talk:Single_login_specifications and the article page itself for some of the current discussion.
The current proposal suggests that "the user under a given name with the highest edit-count" will "Win" that name, and all other users with that name will have a fixed period of time to find a new username. The idea of arbitrarily forcing one user to change names seems bad and anti-consensus to me -- if two active users have the same name, I think they should work out something among themselves -- or be allowed to forego the privileges of unified login. And the idea of using edit counts to determine "winners" is particularly unappealing to me (how about seniority? usefulness of edits? current activity?).
Please check out the current specification and comment. There will be a lot of detailed discussion about this implementatoin at Wikimania in August.