UIpdate from mmo-champion.com: "The next unlock will happen on January 5th, if each wing is unlocked after a month it means that we won't see Arthas before April but if I had to give my opinion I would say that it will probably be faster than that. The 4 weeks are probably here to let people test the rest of the content without focusing too much on raid instances."
So there you have it folks, they're giving themselves plenty of time to harvest those delicious greenbacks without updating the content at all. If MMOs are supposed to be constantly evolving persistent worlds, WoW now wins the award for consistently stagnant persistent world.
The "other content" they're referring to are the 5-player dungeons, but those will just get toasted by the end of the week.
If you want to remain playing WoW, you might want to consider putting a dent in Blizzard's coffers by cancelling your subscription at least until the new raid zone is fully released on normal difficulty.