This is something that I also want to do, but there is a long list of "things to do" before I get to this. My basic idea is to put a wireless card in my server at home, and then one in the car. Every 10 seconds or so, the player tries to ping the server. Naturally, unless I am in the driveway the ping will timeout. However, once I get close enough to the server, the ping will go through. The player will then wait a few more seconds, and try a few more pings to make sure the connection is solid. When it is, it will log into the server (an NT domain running TCP/IP) and through a standard file share, automatically copy any new MP3s to itself. Those MP3s would just be placed into a directory called something like "Copy To Car". The player software will be "dumb", and just blindly copy anything in that directory to the MP3s partition on the it's HD. At the same time, it will copy over a new version of the database that contains all the songs and playlists. It's a simple idea really.
But as I said, this is a long way off. I have more important matters to attend to first.