I like how everyone convieniently has forgotten that Halo 3 launched in the same holiday season as Call of Duty 4 and The Orange Box. How's that for competition?
EDIT: And oh look, a Halo 3 full price expansion puts up 5 million units going up against Modern Warfare 2 and Left 4 Dead 2. The argument that Halo's never gone up against competition before is a myth.