The only reason you'd want to pre-order is if your local store is pulling a midnight opening for pre-purchasers only. Even then, though, they sometimes allow you to enter a few minutes before midnight to pre-buy it, and then get back in line.
Was different with GoW though, at least in Hollywood -- they got it in-store around 6pm and dispensed it to pre-purchasers first, and ended up running out before everyone else could get their hands on it. Of course, GoW was more of a hit than our local Gamestop anticipated, which is why they ran out. Super-doubt that'll happen with Halo 3.