it would make sense to simply sell N cards, they already build them for use in Mac Pro machines
and it would be a little unfair to shortchange us early adopters who have Core Duo machines with no N and removable Airport cards.
Plus I don't see the upgrade from B to G being the same, G to N is a HUGE leap, it's also quite future proof for the foreseeable future
as for selling machines wirelessly enabled and therefore not selling the part separately, maybe, but there will always be a market for people wanting to upgrade, and even repair, maybe they will only sell them to licensed repair places?
I guess time will tell, but here's hoping.