I think a lot also depends on what software comes with it. If Natal on its own is the £25 being suggested by Pachter then obviously that is cheap and will sell loads. Microsoft are going to advertise it like hell, they may even sell it at a big loss. They just want everyone with a 360 to get one and then make the money back on the software for it.
Natal seems to be very mainstream as in it is being reported everywhere. I don't think i have even heard The Arc being talked about here. So i would not be surprised at all to see it crush Arc sales, isnt that what everyone is expecting anyway?