What about the possibility of implementing Compact Flash in the future? Much like what is being used on Digital Cameras and PDA's? The size is much more compact than the PCMCIA/PC Card format and the types of functionality that people are looking for is there (xircom makes an ethernet card for compact flash slots). A nice feature would be copying additional music to compact flash and just add it to the unit when I was ready to leave. One way of overcoming the size limitation would be implementing a majority of the electronics into the unit and having an extension that could be mounted elsewhere within the vehicle (like some of the older remotes for cd players) and a more extreme case would be forgoing the additional hard drive and adding the necessary electronics there (perhaps as an option to the additional HD space for the future?)
Just a thought...