So, I'm from the Midwest and now live in the city. Even though the only time I'm "off road" is when I'm driving up the ramps into a parking lot, I've still got to have myself a real truck. Hell, I helped some guy move an 8' couch with it the day after I got it. These things are useful. Anyway, what does every geek need in their big ugly truck? That's right, a 4-channel surround, 2-sub-woofer, flush-mount touchscreen, USB, Bluetooth, WiFi, GPS, OBD-II in-dash PC.
So far, the closest thing I've found to what I want to build is the Kenwood DNX9980HD eXcelon In-Dash Multimedia Navigation System. This thing has a couple of cons that make it likely not the head unit for me: It costs $1k new ($535.44 refurbished on Amazon); getting OBD-II connectivity still means buying a $90 Garmin ecoRoute add-on dongle; According to Crutchfield, it doesn't fit the truck anyway. (Not that that would actually stop me. Have Dremel, will travel.); and most importantly, it's not hacker friendly.
That said, the thing does have some great features that I want. Primarily, it's the only deck I've found that takes OBD-II seriously. I like having the extra / more specific gages and I like having the trip computer. Most importantly, though, I like the error code analysis. Currently, I use my iPhone and a GoPoint BT1, which works surprisingly well. As I'm getting more into the DIY aspect of things here, I'll likely pick up an ELM 327 because they're cheap ($15) and already have existing open-source software which means writing some more open-source software won't be impossible.