Since the wii is 90% sold out every week, surely its numbers are pretty much guarenteed, the 360 on the other hand could be effected by a major game release in the last/next few weeks, does the 360 have anything major due? The upcoming price cut could make a big difference.
Judging by the current sales charts, i would guess at Aguset the 19th, maybe the 12th if the reports of production being raised are true.