Brion and I are planning a pilot of the CentralAuth (account merging and unification) extension on Wikimedia wikis. The general idea is to enable it for admins only for the duration of this pilot. Admins make good beta testers for this extension because:
* They're responsible enough to report bugs instead of ignoring them * They're more likely to use their first hand experience to write documentation or otherwise pass on their knowledge to others * Allowing admins first choice of username reduces the potential for name-squatting by trolls
Access to global usernames is generally on a first-come, first-served basis. We can deal with imposters when we have to, but it's going to be a pain so it's best if we can keep the number of incidents a minimum.
Also, we hope that limiting it to admins will make the volume of support requests more manageable.
I don't think we'll need to schedule an exact time, or set a site notice or anything like that. When we accidentally enabled it for a brief period last week, hundreds of people knew about it within half an hour. It appears on the first page of your preferences when it's enabled, and word spreads fast.
For general information about CentralAuth, please read the documentation at:
http://meta.wikimedia.org/wiki/Help:Unified_login
-- Tim Starling