Why not go all the way ?
http://www.streetdeck.com/products.php
Touchscreen, Wifi sync with teh home pc, GPS gadgetry, vehicle diagnostics, the whole workz... It's a full-blown media-center pc in a HU form factor. Though having your HU boot-up for 25 seconds will be annoying.
Wonder how well these things work.
The Linux hacker types have similar projekts going on too ...