Announced this evening by Zarhym in the forums here.

A few details:

• It’s going to be a Northrend-centric event, specifically in Icecrown.  This says characters with cold-weather flying to us.
• New linked up daily quests.  Sounds like IQD/SSO.
• Every kind of reward for participation is being promised: gear, titles, achievements, mounts, pets.  We don’t see bags of gold listed, but that’s probably an oversight.
• It’s an Argent Crusade rep-maker, too.  Hopefully, this means some good new Argent Crusade rep rewards.