It seems that with some simple math Kotaku have come to conclude that of all the Xbox 360 owners in the US, over half of them own Halo 3.
While their assumption might not be too accurate it definitely gives you an idea of just how popular the Halo series has become, with 7.9 million 360s sold in the US, and 4.1 million copies of Halo 3 sold.
This shouldn’t be a surprise to most considering how hyped Halo 3 was prior to release causing some to buy a 360 for the soul purpose of playing this game.