GameStop listings are about as accurate as a 14th century musket -- especially when it comes to release dates -- but that didn't keep news site GameSpot from capping the above screenshot. As you can no doubt read, the picture suggests that SanDisk will indeed release a specially-branded 8GB USB flash drive alongside the Xbox 360's USB storage update -- but at twice the normal price for a drive of that capacity. Our red hot rage at this injustice is tempered somewhat knowing there's no concrete proof the $40 figure is correct, but knowing SanDisk (and, frankly, Microsoft's own propensity for overpriced storage) we wouldn't be surprised to see several green thumbdrives pulling a premium at retail next week. Once more for the record: as long as it's larger than 1GB, smaller than 16GB and you format it using the Xbox 360 menus, any USB flash drive will do.
Update: According to Major Nelson, it seems that USB flash drives larger than 16GB will work with the Xbox 360 update, but only 16GB of the drive's space will be available for the Xbox 360 to use. Thanks, willowtwf!