I don't see the appeal of having built in WiFi. If you want to stream HiDef content, plain old g network isn't ideal and it means anyone with n based networks are out of luck. That and the fact that using a cable (where possible obviously) is still the best option IMO.
It would be nice, but not if it means a more expensive 360. A price cut for the console and the adapter would be better IMO.
Well the DS has built in wireless so i dont see how it could make it more expensive, as for the appeal of it, my tv is at the other end of the house from my router and having wifi is very handy.