Even just a compartment inside the HU where you can plug the unit in and then slide it into a foam enclosure. The space inside the HU could be a standard size (biggest unit available) then you could have different "sleeves" that would fit into the big HU space. Add a retractable firewire/usb2 cable and you'd be set to become backwards compatable. The Ipod/Karma/Dell DJ are all small enough that they would fit into 1/2 of the space of a typical DIN slot. The other half could be stacked electronics that would provide the display and control. After you've slid your portable into the slot, flip down the cover (display and controls built into the cover) and it looks like a normal head unit, with added shock protection for your player. Never have to worry about your player bouncing around in a dash slot or on top of the dash.

Actually, this could be a pretty cool aftermarket item. Someone could sell one unit that would fit all the players, just with different adapter cables and foam sleeves. Remember, you heard it here first! Anyone want to start a business