Talk about some pent up demand.  That comes out to 1.5 million pre-orders and 300,00 in additional sales. At an average cost of $70 that's 126 million dollars in sales.  To be honest I was surprised how well Halo 3 is selling considering when Halo 2 came out there where twice as many Xbox consoles in consumers hands compared to Xbox 360.  Just think what will happen when the price of the console gets more affordable and those price consious gamers step up and get their copy.  You could easily see Halo 3 sell 5 or 6 million copies.

Update:  Sales totaled $170 million in the first 24 hours in the US alone.  From that I can tell from matchmaking the game did a few sales in Europe, South Africa and Australia as well. 

Source:
http://www.n4g.com/industrynews/News-70182.aspx
http://www.majornelson.com/archive/2007/09/26/it-s-official-quot-halo-3-quot-registers-biggest-day-in-us-entertainment-history.aspx