Late yesterday evening, users of the latest build (1.3.2) of the Adium multiservice chat client found themselves unexpectedly at a loss -- connections to Microsoft's MSN IM service stopped being accepted as of about 8 pm Pacific Time, according to a post on the Adium blog. The cause appears to be some kind of change on the server side that is blocking Libpurple (the underlying Adium communications library) clients using version 15 of the MSN protocol, so Adium isn't the only app affected.
Users are advised to either roll back to Adium 1.3.1, which uses an older library that's still working, or use the official Messenger 7 client instead. The Adium team says they'll update the post when they have more details. Update 4pm ET: Reports are that Adium's MSN connectivity is working again.
Thanks to everyone who sent this in.