I am fascinated with this entire MP3 car concept. I've been looking at this thread with a keen interest and will purchase the chinese ELM327 tonight, it's so cheap, if it works at all, it would be hard to fault it.
Regarding the question around using this in .Net, I'm a developer with reasonable .Net and vb6 skills, so I think I may be able to help the programming issues here and would love to contribute. Bring the questions on.
First off, you can probably use the VB6 code straight away in .net without a conversion. All one needs to do is build the class as a DLL using VB6, and then you can call those functions from .Net without converting it to something .Net will compile by adding a reference to the .net project, do browse and find the already compiled DLL. I agree with the previous posts, conversions to .Net for low level stiuff like this will almost certainly result in disaster, and since you don't want to redo all that great work in this class, build it as a library or write a simple wrapper class in .Net if you are familiar with that concept.
I've also written custom gauage classes for displaying data like this graphically including customizable redline values, refresh rates, decaying historical depictions of previous values over time (imagine a fading needle retaining your highest rev, like windows media player does with the equalizer visualization).
I've also dabbled in engine management computers for fords ECU using EEC Tuner hardware and software. I can't claim to be an expert but I certainly understand the concept of fuel trim, injector pulse width, timing, and I have reasonable development skills.
I should be able to provide even more insight once I get this ELM327.
This was while back and I haven't had any time to do any updates or anything. I am still using the same codes from then until now. MPG calculation seems to work well. HP and Torque has some errors. Sometimes init fails.. =)
Hope it helps.