Nikon D3100 is already listed in Best Buy's inventory system. The SKU is 1222817, the description reads "Nikon D3100 DSLR w/ 18-55mm v", availability: 9/18/2010. No other information is provided other than the box dimensions. BestBuy does not create SKUs based on speculation or rumors, however price and availability date is always subject to change.
There is still no listing for the Nikon D90 replacement and that makes sense since the official announcement will be a month after the D3100. The D3100 is expected to be announced next week and usually new products are available in stores a month after their release.