Ditto. I went looking for a replacement for the crappy nav Toyota offers for the Prius and was surprised that by now there isn't an Android HU available anywhere for any car.
What I mostly want is the ability to do the things that factory headunits already do, only better because the software is customizable. (As opposed to having a general purpose computer to play solitaire on at 80mph, like so many in this thread imagine.)
For one example, the 2010 Prius nav requires an expensive XM subscription for traffic data, map updates cost $200/yr, and it does a crappier job of routing around traffic and estimating arrival time than my 2-yr old $200 Garmin handheld with lifetime free traffic data. So I want to download Garmin or TomTom and run it instead of the crappy Toyota software.
For another example, the interface for playing iPod stuff totally blows IMHO. I'd like the option to change it or download a different interface, but I'm stuck with something designed for other brains.
Once you've used a factory unit and seen how much better it could be at its own job, you'll understand why there needs to be an Android headunit platform. Car makers can only provide a one-size-fits-all, lowest-common-denominator, long-lead-to-market, non-upgradable system that is a waste of potential. Its time they turned the job over to a wider market and leveraged all the programmers they don't have on their payrolls.