With the announcement of Cataclysm, there is a lot of misinformation floating about regarding what parts of lore Blizzard is "loreloling." But the truth is that much of the new lore that has people up in arms is actually very well established.
Part II: Awrooooooooo! Were-have I Already Done This Gag? The Worgen.
So I'll admit that my personal predictions / beliefs about the worgen were off. I didn't think Blizzard would go with the shapeshifting race due to their previously stated desire to give every race a distinct silhouette. However, Cataclysm is re-introducing Gilneas to Azeroth's geography, and with it are coming the shapechanging worgen.
Strangely, there doesn't seem to be that much controversy over the addition of worgen themselves (most people that I've spoken to, me included—yes, I speak to myself sometimes—think they're cool). The lore controversy seems to revolve around the idea that worgen can become death knights.
"But Thar! If Cataclysm happens after the events of Acherus, then there CAN'T BE worgen death knights!"
Well, yes, if you're assuming a linear timeline, which World of Warcraft does not exactly have.
"But Thar! ... wut?"
I've explained this a few times before in other places, but I'm not sure how in-depth I've explained it here. You see, World of Warcraft's plot progresses in a hybrid fashion. Huge world events happen on a static timeline and happen for everyone. The best examples are the opening of the Dark Portal and the plague event prior to Wrath of the Lich King.
The "smaller" events (and I use the term loosely) happen on an individual basis per each player's personal quest progression. In other words, if you're a level 1 draenei, then the game's plot explicitly states that you JUST woke up one month or so after the Exodar crash, despite what other players in the world are experiencing. To you, Illidan Stormrage is still alive atop the Black Temple, and Kael'thas hasn't had his setback.
Because of this parallel progression structure, the precise timing of certain events is unclear, and many things are "pancaked" on top of each other. So, for example, the events of Acherus (the death knight starter quest) happens before the events of Outland. This is because death knights start at level 55, and they're expected to experience the events of Outland after they defect from the Scourge. This is further reinforced by the Lich King's claim that the Ebon Blade defected "months ago" when you encounter him atop Naxxanar in the Borean Tundra (a part of the Alliance side quest dealing with Thassarian and his sister).
In other words, even though you may have been leveling your level 58 death knight at the same time you were leveling your level 71 main character, those two timelines were occurring months apart per the world lore.
Is it starting to make sense yet?
No? Okay, so how about this: the non-wretched version of Kael'thas is still the final encounter in Tempest Keep. But Kael'thas is also the final encounter in Magister's Terrace. Are there two Kael'thas's out and about? No. It's the same Kael'thas, just in different points of the plot progression. The events of Quel'Danas and the Sunwell Plateau happen after the events of Tempest Keep.
Alright, with THAT out of the way, we can now once again look at how the worgen fit into this timeline. You see, since the worgen will be starting at level 1, then they will experience all the revised 1-60 content and storyline that the other races / classes experience. This means that Gilnean worgen are retroactively added back into the timeline. And because of this, Gilnean worgen CAN, theoretically, have been on Acherus.
Not good enough?
Alright alright, so I'll give you the easy way out: the Wolfcult.
Yes, in Northrend right now, barring any parallel personal progression plot mumbo-jumbo, we have shapeshifting worgen allied with the Scourge. Indeed, we have the Shade of Arugal once again commanding his children. If we have Scourge allied shapeshifting worgen... then we can have worgen death knights.
Granted, Cataclysm will bring about its own anachronisms, I think. But I'll wait until we have more information to tackle those concerns. Until then, just understand that there is no exact timeline of detailed events for World of Warcraft until Blizzard chooses to canonize the events in another medium (books, comics, manga, etc).
Makes keeping up with lore kinda exciting, doesn't it?