MMO-Champion reports that the first 4.1 release candidate has just been deployed on the PTR. Boubouille is predicting that it will go live the last week of April or first week of May.
The new raiding tier isn't coming until 4.2, but 4.1 will reintroduce Zul Gurub and Zul Aman as 5-man dungeons. Their heroic versions will drop a new ilvl 353 epic half-tier that fits between current heroic dungeon and normal raid rewards.
What else is in it? Check out MMO-Champ's 4.1 patch recap post.