We don't have much solid information on what's causing it, but the theory so far has been an ISP issue. I'm not very tech savvy when it comes to that sort of thing, so I can't really say how true that may or may not be. However, our buddy Belfaire seems to be doing what he can to take care of it. He just posted that they think they've found the issue and may be rolling out a fix.
So, you guys with characters stuck in limbo? Hopefully your problems will go away very soon.
Updates behind the cut as they come.
Belfaire confirms that the characters-stuck-in-limbo issue is on Blizzard's end. There is no resolution yet, but the following message just popped up on my home server. There may be a fix rolling out now, and there might not be. No word from Belfaire yet that I can see.