I preordered halo 3, but only because I found a misticketed item at gamestop, and the employee said he would sell it to me at the price on the item if I preordered halo 3 and some other game, I think it might have been assassin's creed.
It was a win/win. I got a 10 dollar xbox 360 bag, two preorders of games I would probably end up getting anyways, and he got 2 preorders, which I hear is the only way gamestop makes money.