This video went up a few days ago onto the marketplace, but here it is for those of you that don't have a 360 or just would rather see it on your PC/Zune/iPod/PSP...
Here is the official description (from Major Nelson's post):
"In the second Bungie “Vidoc” we take a long hard look at the ferocious new antagonist for Master Chief, the Brute. Now that a tentative alliance has formed between Humans and Elites, the fighting focus of Halo 3 switches to the monstrous Brutes. These tenacious foes have improved graphically with the advent of the Halo 3 engine and the power of the Xbox 360, but more importantly, their tactics, weapons, vehicles and AI have all followed suit, making the Brutes the worthiest opponents the Chief has ever faced. See how the designers, artists and programmers set about the herculean task of replacing a fan favorite – the mighty Elite – with an unforgettably ferocious adversary. See how Brutes behave in packs and as individuals with brilliant new animation and AI. Take a first look at the new Brute graphics, weapon and armor designs and finally, get the first ever public glimpse at Halo’s legendary Campaign.
Frankie gives you more details at Bungie.net"
And for those of you who notice... yes, this was widescreen and yes I cropped it down, but this actually made the in-game footage fill the screen (it was 4:3 in the original video) so I thought it was a better overall result 
And if you really want the highest quality... you can grab the 6Mbps original as well, 640x360
