Halo is trademarked: http://www.thewhir.com/web-hosting-news/inld032103 I guess it depends on how you're going to use it.
It's allowed, as long as you don't build a site about the game with the same name. Or about anything that's trademarked. You can buy I-Like-Halo.com, but you build a site about "A luminous ring or disk of light surrounding the heads or bodies of sacred figures, such as saints, in religious paintings; a nimbus.". That's okay. But if you make it about the game, then it's not allowed.
Exactly. DMI summed it up perfectly. I wanted to do this for a product I wanted to promote but someone highly recommended I don't do it. It makes sense though.