...and to answer my question, it turns out the October 2005 issue of Circuit Cellar has an article called "
Fuel Consumption Gauge", which uses the OBD interface.
I just received my copy yesterday so haven't had time to digest it yet, but it least it appears very likely to be possible.
A Google search turned up a few hits of people suggesting using a gumstix+OBDII interface to make an in-car system.
Has anyone here actually done this?
If not directly to something like the Robostix, any suggestions on interfaces that work?
If not specifically Gumstix, with Linux machines in general?
By virtue of being RS-232, this looks like one possibility: http://www.vehicleinterface.com/product.asp?sku=LDV6S
If you don't know what to do, do something.