It will definitely be an MMORPG
Warhammer 40k MMORPG will be built by Vigil Games and published by THQ (who brought us Warhammer based RTS masterpieces such as Dawn of War).
The game is already licensed for development by Games Workshop.
The game is planned to be released in North America, Europe and Asia.
There is no time scale for development as of yet
The Austin based development studio, Vigil Games, is developing the Warhammer 40000 MMORPG under the watchful eye of David Adams, formerly of NCSoft (Guild Wars)
THQ believe that Warhammer 40k is the stronger of the Warhammer franchises, and therefore one can assume they predict that the game will be more popular than WAR: Age of Reckoning
The game will be set in the the 41st millennium, after the Horus Heresy
Game mechanics will employ techniques from the table top game such as cover, suppressive fire and flanking.
All races important to the lore or Warhammer 40k will be included in the game. Presumably this will include races of all playable armies from the table top game
Characters will be highly customizable
Players will be able to explore a variety of different landscapes such as cities (of all scales, types and sizes), exotic alien temples, chaos shrines, deserted battlefields, mysterious ruins, ancient structures, and drifting hulks in space.
Some planned NPCs include imperial citizens, chaos sycophants, xenos, daemons from the warp, eldar craftsman, oge raders, ancient killing machines and many more
There will be solo, party and PvP missions available, and you will not be forced to group for them.