Here's the problem with a WH40k MMO:
People who just like MMO's are going to keep playing WoW, Aion, EQ, whatever - the game they're already familiar with, already have formed loyalties within, and already know that they like.
The people who like WH40k are going to play it - and then quit, because Tyranids would NEVER work together with Chaos, Space Marines would never willingly operate with Eldar (or xenos of any kind for that matter), IG should die WAY faster than they do (i mean, seriously? my khornate berzerker hit that guy THREE TIMES with his axe! an IG shouldn't be able to take ONE hit from a khornate soldier and survive!), they don't have my favorite but ubscoure and underrepresented faction of Space Marines available as a core faction, and there's no 'Rape' option for slaaneshi chaos marines.
The people who just like futuristic fantasy games are probably going to play it for a month or two, and then bail back to the Crysis 2 multiplayer, Microsofts new Halo 4: The Exploitation, whatever new futuristic game Valve is apparently coming out with, and so on... assuming they aren't too busy playing that new Star Wars MMO that's apparently coming out soon... or playing what will probably become a F2P-Freemium Star Trek Online.
In order to get any real kind of success, THQ is going to have to revolutionize how MMOs are played - probably doing something more along the lines of an MMORTS - to keep the core WH40k fanbase happy, and even then I really doubt that the actual fanbase for WH40k, vocal as it is, is actually big enough to sustain a game that cost $50 million at launch...