Battle.net accounts, and from all indication they don't all seem to be for the same reasons. Tom may not have the same issue as Sally who isn't necessarily getting the same error message as Jimbo. As much as the problems are annoying me, the more widespread problems tend to clear up pretty quickly. Usually. For some of the odd, lingering issues you may run into, Blizzard's Support page has added a section for Battle.net connection issues within World of Warcraft with some common solutions. Note that these won't necessarily fix every issue that crops up with the login servers, but it should solve some of them!
The most common solution that seems to work out for people is the suggested edit to the realmlist.wtf file, using something like Notepad. Look for the Data folder within your World of Warcraft folder, open it up, then open Realmlist.wtf with a simple text editor. Check to be sure all four of these lines are contained within: