I know what you are saying, but as a consumer who is not afraid to spend money (I have a 64gb 3g ipad) and also as one who is not afraid to have the monthly fee (I pay monthly for 3g) this would definitely be a deal breaker for me. If I wanted wifi alone, or if later on I wanted to cancel my 3g but keep wifi, but the carrier said I would have to pay for wifi, you can bet I would steer far away, and once again it's not a financial issue.
Changing your stance from equating paying for the hardware feature of wifi to paying for 3g service to saying since the device costs so much consumers won't care isn't cutting it. This will put a big crimp in the sales of the unit and I'll bet they stop doing it very shortly after it gets released. There are just too many choices out there, especially upcoming choices this year. I also see a perfect opportunity for Apple to market the ipad2 as being the tablet where you don't have to pay for wifi, kind of like their PC vs Mac commercials.
Well it isn't like you have to pay the monthly fee every month to continue using wifi. You just need the one month 3g to get the device activated and then you can cancel and use as much wifi as you want at no cost. It is more of an activation fee than it is being charged for wifi.