The Aether mod, one of the biggest and most downloaded mods ever, which also won the Diamond Pickaxe award last year, is being updated by The Aether team for 1.4.x with multiplayer support as we speak. They had this to say on the matter via their Facebook page:
The team has started updating as of yesterday, this is what we have so far (picture above) There's a lot of generation content missing in the picture, but we're working as hard as we can. And yes, this is all working on a server
So when can you expect it to be updated? Even though the team said nothing about a timeslot, if past update periods are anything to go by, you might be enjoying this mod by the end of the month. This is just a guess based on past experience, so don't take it as confirmation.
Stay tuned for more info on the Aether mod, and everything else Minecraft related.