I ditched my smart phone a while ago and have reverted to a landline at home and a prepaid flip phone for emergencies on the road. Going retro-tech
I'm liking the Android idea more and more. I like that I can integrate everything into it since it's just an android device.
I already look at my rearview mirror fairly regularly and to be honest, I really don't need to look at a GPS all that often as voice commands are usually enough with a quick glance here and there.
Having it all the way up on the mirror could be inconvenient when you need to type something, though. Speech to text or one of those tiny peripheral keyboards, maybe?
Being able to integrate a dash cam DVR, music, rearview camera, OBD real-time information, etc. just seems like it would be very worthwhile.
How about a 7" Android tablet as a HUD with a small keyboard and trackball hidden or integrated somewhere? Ok, too much coffee today

I need to go build something before my head explodes.