On topic:
How about a green lightbulb with a tree inside. Perhaps not fuel efficiency, but maybe energy efficiency.
As for the other "discussion" it's not like he asked anyone to come up with a fully-fledged design blue-print. To me it seemed more like he wanted some help brainstorming...
Coming up with an original idea is often the hardest part of a design project. Those of you who think designers are being unreasonable about helping him with a concept have no idea of how difficult or time consuming that can be. Just take a look at all of the ideas offered so far... they have all been very generic and cliche'. Of course if you don't care about the quality of your message, then that might be okay with you.