Unless Apple will be contracting the supposed new plastic back covers from a different injection plant than now makes the fronts, this rumor simply is not true.
Apple explored the white plastic back idea, even going so far as to create functional prototypes and obtaining firm costing from the plastic injection shop last Fall. After that, there was no further activity toward a plastic back.
Interestingly, the stamping plant that makes the current nickel-stainless back had dramatic reductions in production orders in January through February. But, that has now passed, and production has been ramped back to "early 2002 levels" again on the shiny pack covers, for "over a month.".
