While some people are positing that this is a merged battle.net account problem, it's worth noting that we also know of many people, including a couple of our bloggers, are still able to log in fine with a battle.net account. At the same time, some posters in the aforementioned thread are saying that they can log on fine when they use an unmerged pre-battle.net account from a family member or roommate.
Once we get a clearer idea of what the problem is or when it's been fixed, we'll let you know.
UPDATE, 10:30 PM EST: Okay, This is Daniel again, and I can log on with a battle.net account. This is the same one that wasn't working not 10 minutes ago, for the record.
UPDATE, 11 PM EST: It looks everything is working fine again. Play on!