A path that I've gone down, maybe it does not belong here. But here it is anyway.

HP 6515a (PDA + built in GSM phone + GPS receiver)
2 Gb MiniSd with loaded music + gps maps (total theoretical capacity is 4 +4 = 8gb)
Car holder + Car charger for HP
Tape adapter
Cost $250 (used device off ebay).

Custom written audio player by me (not necessary, you can use anything)
iGuidance PPC v3.0 (can use others... Tomtom6 for example)
Cost $120

See the pics + action video here

The main advantages I see of doing this instead of the whole lcd touchscreen + carpc shabang is cost and revert to stock with ease.

What can I say it works! And it is my phone too! The absolute best part is when you are driving around listening to music and a phone call comes in... the music auto mutes, the phone app comes up, i hit 'answer' and the caller's voice comes through the speakers. When the call is over, the music auto un-mutes!

For the price I paid, it is great. The only disadvantage is that I have to set it in place and plug the audio + power in everytime....but this is a 10 second thing. But no power-up time, and no shutdown suspend troubles.

Anyone else done this? I would love tips. Only one i've seen before is the dell axmizer guy. That was my inspiration.