Article:
D&D 4th ed: Space Marines!?

I love the idea of Warhammer 40,000 running head first into the Dungeons and Dragons world. I imagine the imposing bulk of an Imperial space vessel plunging out of the warp to sweep into orbit around an ancient and forgotten colony world, cut off for thousands of years due to a near perpetual warp storm, it's mystical energies bathing the planet for so long, magic has become an everyday part of the world and it's people.
Stranded for so long, cut off from the rest of the universe, the races which were once mortal enemies have formed strange and unholy alliances.

Eldar, now calling themselves Eladrin and Elves, have found a home among the primitive cities and settlements of Humans.

The Squats, calling themselves Dwarves, have created vast underground realms and mountainside fortresses, their kingdoms ancient and fallen into decline.

Orcs are much the same as anywhere else, being savage and barbaric, yet strangely less numerous here and able to somehow interbreed with the other races, which might explain why they are not able to sprout like fungus in their crude camps.
Other creatures, things of clearly demonic origin, are walking the streets of Human towns, calling themselves the Tiefling race.

And there is more, so much more, that would set any Space Marine's blood to a low boil at the mere mention of it.
As outlandish as the idea of a creature from the realm of a deity, or a mewling abomination from the Far realm is; an adventuring party running into a squad of fanatical Space marines is not really that strange in comparison, and I can assure you that your players will react with a suitable sense of hilarity and dread when facing the greatest warriors of the far future, suddenly dropped into fantasy central (and really not liking what they find).