It was mentioned somewhere in a different thread, though they just assumed the top would be black plastic as coming in iPod nano colors would mean that Apple stores would have to carry at least one of every color or would possibly lose some sales due to them not having the specific color the customer wanted at the time.
Nope, go the way of dell, interchangable tops. snap it off, snap a new one one. even the Dell Hybrid Studio have interchangable sleeves (for $20 each).
Macbooks in store would have no top plastic on them, and they would snap on the desired color at time of purchase.