FYI, its not an ad for Halo 3, it's the first of series of short films directed by Neill Blomkamp, with WETA Workshop and Origami Digital LLC. Yes it mentions Halo 3, and it may be used to promote it to a small degree, but it's not really an ad.
It can be downloaded in HD from Bungie's website: http://www.bungie.net/News/content.aspx?type=topnews&link=ethreeblowout
Scroll down, past the screenshot.
The setting is the Misrah Armories factory on Mars. The warships are thought to be unfinished Marathon class cruisers.
I would disagree with the statement that the marines look like "they're in line for a comic-book convention" I reckon they look pretty good, though the bit at the end where a plasma bolt hits the ground next to a marine was a bit tacky.

Archaos said what I was going to say, and that is that this is not an ad for the game in any way. This is from the team that was originally developing a feature length Halo film, but now the project has shifted to short films, this one apparently called Arms Race. Great ad, anyway.

Haha, I can only imagine if the Halo movie DOES turn out to be like this...

BTW, in the Halo universe, those weapons aren't 22nd century but rather around 2550 so 26th century.