If the beta is half as awesome as that video looks, who knows what numbers it could put up?
It sold 3.8 million worldwide first week on 11.5 million consoles. So 1 in 3 bought it week 1.
The 360 should be at around 41 million consoles or so by then. Even if the first week attach number for Reach drops to 1 in 6 - so half as much - it moves 6.8 million units.
It could be huge, huge numbers. Again - the beta has to be great and the hype machine needs to be ramped up in the summer (like it's not going to be?) but there is a possibility that this sets all sorts of records. If MW2 can move 4.9 million world wide with another console as an option?
There's absolutely no reason Halo: Reach can't do 8 million in its first week if its advance buzz is good.