It’s official: the "cash for clunkers" automobile rebate program came to a close yesterday. But did the government act too soon? While the removal of older and less fuel-efficient vehicles gave a much needed boost to the car industry and the economy in general, it’s only natural to wonder if trade-in incentives will be offered again when the environmental benefits will be even greater. When will this occur? With the introduction of plug-in hybrid electric vehicles, commonly referred to as PHEVs.