UPDATE - The YouTube trailer is actually not a leak, but rather pulled from this GameSpot exclusive.
ORIGINAL STORY - Aliens: Colonial Marines has gotten its debut trailer, featuring confused and stranded marines who are then inevitably picked off and devoured like fleshy little hors d'oeuvres. The game appears to feature a not-insignificant amount of head-biting, but the aliens spill their share of green blood as well.
The trailer hit YouTube (via OXM), which may mean that Sega didn't quite mean for it to get out so early. If you want to see some hot alien on human action, you may want to hit the "Play" button toot sweet.