Ubisoft's uPlay, the online service largely used for validating the digital rights management on PC games, has been down for much of the day. If this is planned maintenance, Ubisoft didn't do a great job of communicating that to its users, who are ripping the publisher a new one on its official forums.
Users have complained that they are unable to log in to the uPlay PC client, but they still are able to reset their passwords. One user posted the trouble ticket he opened about the outage, and the reply included a link to instructions for deleting one's account.
This is an especially unfortunate time for such an outage as it occurs in the middle of the Steam Summer Sale. A lot of the forum anger comes from people who picked up a new copy of a game today and simply cannot play it. Some are offering workarounds to boot the game in offline mode, but most are venting their understandable displeasure.
If this is maintenance as opposed to an attack or other cause of the downtime, there's no estimate of when uPlay will return. I pinged an Ubisoft representative to see if we could get some answer as to what is going on.
Cannot Log Into uPlay [Ubisoft Forums]