Ok, so one of the downsides about using your mobile phone for driving directions is that it can't have a comprehensive set of maps stored like a regular GPS unit would. How about some sort of external hard drive you can hook into? It would like you have extra movies, map data, whatever. That way you can pre-load a lot at home before you get on the road.
Personally, I'd want to be able to use my phone to download over Wi-Fi, but I have no desire to pay to download over the EDGE/3G/4G network.