The demo for Space Marine is out today(ish). its not up yet on the Xbox Marketplace but should be later. Its out or selected people on PC, and assume the same will be true for the rest of the PC world and PS3 today.
Comprising of 2 missions it should be awesome!
[url]http://www.youtube.com/watch?v=nb50aAFiOpM[/url] - A walkthrough of the demo, voiced by Totalbiscuit, a professional reviewer/40k fan. It just looks spectacular
I am just going to be shouting "Purge the Unclean! Burn the Alien! Death to the enemies of the Emperor!" all the way through this game i'm sure
Edit - Its up on Xbox marketplace now. Assume its up for PC and PS3 as well