Blizzard is aware of the issue, but currently has no ETA as to when it will be fixed.
Update: Blizzard poster Claudiog says that the issue seems to have been resolved. For my part, I am finally back online and raiding as of 11:12 PM EST.
Update #2: seems that problems persist and Blizzard is asking affected players to help them lock it down. If you can post your ISP and a traceroute to that forum thread you should probably do so. We're also getting widespread reports from EU players that there are several kinds of problems with their login servers as well. We have an image of the European client's Breaking News box after the jump.
Update #3: There's now a list of affected servers. We'll include the entire list after the jump: still no ETA on when the issue will be resolved.
September 9th, 2009 10:00AM PDT
The realms listed below are currently experiencing intermittent connection issues. At this time we have no estimate on when it will be resolved, but we are actively investigating the situation and will provide more details when they are available.
Altar of Storms
The Venture Co