with 4g USB memory sticks and 8g USB memory sticks getting so close in price at a retail level, it can't cost Apple much more than a coupla bucks to double the memory to 16gb.
i'd be guessing sometime soon they will plug the obvious gap (it's a big jump from 8 to 32g).
and a while later, as costs continue to decline, convert the 16g device over to the new faster internals.
it's a more gradual "refresh" - like Sony used to do with their Walkmans years ago. eventually you add enough value to get people to upgrade as well as gain newbies to the fold with an enticing entry point.