I'm surprised by the average retail price that NPD is reporting for Wii last month. At $ 218 it indicates close to 300k units would have sold at the new price, which means it sold only around 160k units in the first 4 weeks and then another 300k in the last week. So it seems NPD has Wii dropping a lot lower in the weeks leading up to the price cut and then a much larger spike afterwards, at least compared to Vgchartz. I guess it is possible taking all the rumors of a price cut into account.
http://www.gamasutra.com/view/feature/4172/npd_behind_the_numbers_september_.php?page=3