Halo 3 on the Xbox 360 apparently has over four million preorders already, which if true would mean that it is the most preordered game of all time.
XE360.com report that Gamestop officials conveyed this information to them, which they then verified at various Gamestop, EB Games, and Funcoland stores in the United States. This eclipses the number of preorders that Microsoft reported three weeks before the release of Halo 2 - 1.5 million. It also means that Halo 3 will likely outsell Gears of War easily which has only just passed the 4 million total sales figure.
So as long as most of these people follow up and actually purchase the game, we should have a new record on our hands - not to mention the fact that there will be lots of preorders sitting at other stores outside the Gamestop empire.