I looked it up but found nothing on it.
Free republic looked at it in this thread.
I think the low energy consumption of super bright LEDs make them the best option now though, especially if you were to put the rectifier and filter into the fixture and make 12VDC bulbs that would be interchangeable with automotive use LED bulbs. There is already a 12VDC rail on the USB buss and there a ton of small devices using 12VDC converters, so it seems to me a ubiquitous choice.