For some players, this weekend's adventures has had a distinctive theme - downtime. The same twenty realms that have been having problems since Friday are offline again tonight. Some maintenance was done on Saturday morning and afternoon to fix errors with the in-game mail system - but as to what's caused this most recent maintenance, there's no word. On the player side, symptoms were similar to Friday night's trouble, with an inability to rez, zone in, or use skills, peppered with occasional instances of falling through the world. While there's no official announcement, affected realms seem to include the original twenty problematic realms: Andorhal, Anetheron, Archimonde, Black Dragonflight, Dalaran, Daivengyr, Dentarg, Duskwood, Executus, Haomarush, Khadgar, Mal'Ganis, Norgannon, Scilla, Steamweedle Cartel, Thrall, Turalyon, Ysera, Ysondre, and Zuluhed.
Update: While the servers seem to be online Sunday morning, players are plagued by difficulty/inability to log on, as well as severe in-game lag (several minutes at times), inability to zone in/out of instances, empty mailboxes, random disconnects, etc. (Did I miss anything?) There are no updates on the fourms, and the message on the login screen is still from yesterday afternoon.
Update 2: Blizzard informs us that these are latancy issues that they are working to resolve.