From Microsoft: We know that people like you love our games and sometimes want to use things like gameplay footage, screenshots, music, and other elements of our games ("Game Content") to make things like machinima, videos, and other cool things (your "Item" or "Items"). We'd like to make that easier for you. So long as you can respect these rules, you can use our Game Content to make your Items.
Microsoft have laid down new rules which prevent players to make profit through videos and what not from Microsoft titled games including Halo, here is a video made by HaloReachTutor explaining these new rules:
To see these new rules yourself http://www.xbox.com/en-US/community/developer/rules