I, too, would love to ditch my entire Nav/Bose headunit in my '04 but have a feeling I'm going to have to buy a new (or different) car first. I once met an engineer at a company that makes bluetooth extenders for OBD-II, like those at obdkey.com. I often wonder how much information going to the nav unit can be grabbed via OBD-II, and how useful the data is that is not on OBD-II (such as the low oil level I should have known to check!) If you can get all the useful data via OBD, then there's no reason (to me) that I shouldn't be able to rip out the nav and 6-disc changer and definitely extend the useful life of the toys in my car.
My ultimate goal is a double din touchscreen that uses a laptop SSD drive for music and videos as well as a nav that gets lifetime free updates, such as Google Earth or Google Maps or more preferably as an old open source guy, OpenStreetMap. Heck I would even be happy if someone reverse engineered the Zenrin DVDs and published updated files with OSM data. Anything (legal) beats the $180/year they try to squeeze out of you. Sorry, but as a long time OSM contributor, that just burns me up. Every time I enter a destination that I haven't updated in a while (or just drive somewhere new), my nav should tether through my phone to download the latest maps for the area and 5 miles in each direction off the route I need to take to get there. Once a week whenever it finds open wifi, it should check for new updates to the areas it has already downloaded. It should also automatically store GPS tracks where the tracks differ greatly from the downloaded data and upload those tracks in the weekly sync-up. Heck, if you want to keep it proprietary, make it a $20/year subscription but make the map data auto-update over wireless whenever I bring the car to the dealer for any reason. Why is this so difficult?!?!