I know it's not exactly an exact science, but I rang kool n quiet and chatted to the guy that sells the adaptors and he stated that most people that buy them do indeed use them for car pcs. He said that like most car chargers and adaptors (Nokia, PDA etc) it is recommended not to be used when starting the vehicle, but he also said that noone has had any problems doing so uding the adaptor with the 55w dc-dc PSU.
I think I may go for it and see what happens - it's cheap enough that even if it blows something, it'll be cheaper than splashing out on a opus or so (Yeah, so i'm a skinflint, okay?) :)
altho if something blows, itll likely take the psu, cpu and what ever else is hooked up to it.....