Not to "burst anyone's bubble" but wouldn't that be copyright? I mean, we already have Master Chief's armor in Duke Nukem Forever. Where the chief's armor was rudely insulted.
For me personally, I think Halo's weaponry should stay where it is. Not in Left 4 Dead. Well, on second thought, nah. Maybe as an Easter Egg on one level maybe.
They could allow enough to have Halo's Assault Rifle. Now maybe that'd be considerate. Well, this is just my opinion but...