That definitely sounds like an idea. I've got a Duemilanove that first got me interested in Arduino projects, that I haven't really done much of anything with. I imagine I can use the same sort of signal-filtering setup used in the MPGuino for VSS to "safe up" the signal? I'm really not that familiar with the electronics side of things, although I've been dinking with electronics projects all my life, I never had any formal education on the matter... I'm more a computer programming/repair kind of guy. So I can probably do the code once I put my nose to the grind-keyboard
But I take it 160 baud ALDL isn't all that popular, even for GM... if I did end up doing it, it might have to be a one-off thing for each car's computer, since the different ECMs put out different bits of data in different ways, and to keep the code small, the value positions and interpretations (such as reading some gibberish integer as 95.4 degrees F) would have to be hard-coded for each system.
Sure would be nice to have, though. Definitely gotta go pick up my Arduino from my friend's place... been sitting in a box over there for months