ARINC 628 
http://en.wikipedia.org/wiki/EmPower

I want to install one of these in my Vanagon. Can't find anything online. They are 15 Volts DC and four pin. Apple sells an adaptor for these for use on airlines. Unlike the Powerlet type, they are low power. Note that though EmPower is what they are referred to, the EmPower Classic as sold by the company that owns that name, is a nine pin connector (not to be confused with their 110 AC EmPower line). Hypertonics http://www.hypertronics.com/PDFs/ARINC%20628.pdf makes the four pin type. Apple's adaptor is for the four pin type.