This is almost exactly what I want to do.
I've been waiting for something similar with android 2.0 and 3g built in...because I have a droid which is awesome...and the 2.0 + google nav is awesome. Android in the car is reaching its potential for being a perfect carPC platform (or a very viable alternative to windows) IMO.
You mention that you will tether 3g over the wifi eventually, but will you be able to install google nav on a device that doesn't have native 3g built in without rooting?
I'm really liking your install. If you ever do the 3g/tethering bit, keep us updated on how that works out.