Network outages started plaguing the realms late Saturday night, with connection issues across all US realms. It appears to be a network issue, preventing many, but not all players from connecting to the servers. (So if you were wondering why you were the only one who showed up for your guild's late night raiding session, that would be why.) The forums are offline as well, so the only official information we have from Blizzard comes from the breaking news box on the login screen, shown right, which gives us no ETA but thanks us for our patience.
Update, 1:22 AM EST: The breaking news box has been updated, telling us that there's an issue with AT&T's network. Blizzard is in contact with AT&T and waiting on an ETA for resolution. We should have an update on status within the next 30 minutes (as of the time of this writing, quite soon), but for those of you on the east coast, this probably means it's time to go to bed.
Update, 1:28 AM EST: Though there's no official update from Blizzard, authentication is working for at least some players. (Read: my Shattered Halls group is back at it. You can 3-man Shattered Halls, right?) The realms haven't been offline through this period, so you should be able to pick up right where you left off. (Presuming you can get back online.)