there was somebody's maths floating around the net, cant be bothered with the link. Essentially it was based on $170mill US from MS divided into the unit proportions from a major retailer (standard, LE, Legendary) which estimated 2.48 million copies on day one. Google it if you want a link. Rumours are saying the first shipment was 4 million, but here in Australia I know some stores already needed a second shipment.
starcraft - Playing Games = FUN, Talking about Games = SERIOUS