It will be interesting. I have ABSOLUTELY NO DOUBT that it will be the largest single day in the US. However, the candle that burns twice as bright, burns half as long. Not saying it won't continue to sell well, but if everybody picks it up DAY 1, it has nowhere to go but down from there. Usually, most games have adoption rates that range from day 1 to day 90, which is why GTA: VC sold 1.5 in Oct, 1.5 in Nov, & 1.5 in Dec. Madden similarly holds a large monthly sale rate for the first several months.
Halo 2 will probably end up being the most frontloaded game of all time in the US, which isn't to say that it won't sell spectacularly. Just that a disproportianate percentage will have gotten the game on day 1. Much like FF & DQ in Japan.