[UPDATE: Nethaera reports that Blizzard "encountered a serious error" with the listed realms and are working towards a resolution. Unfortunately, there is still no estimate for when the realms will come back up and more details should be available at 5:30pm PDT. The situation has a small silver lining, however, as Blizzard is considering "appropriate compensation" for the additional downtime experienced by players on those realms. Stay tuned to WoW.com for further updates.]
[UPDATE 2: Nethaera updates her post to explain that the network improvements intended for all realms resulted in a connection error which has taken some time to identify and fix. While she notes that the connection error itself has for the most part been resolved (which is why some of the affected realms now show up on the realm list), the affected realms have yet to undergo the regular weekly maintenance and patching procedure. This means a much, much longer downtime than anticipated. Nethaera hopes to give further updates by 7:30pm PDT. Time to reroll, I guess.]
[UPDATE 3: Looks like they're doing a little more work tonight on all realms to address a few issues. More information in our post here.]
We'll provide updates on the "issues" as they come.