I'm only speculating, of course, since there isn't one (in production, at least)
1st, the data drive is pretty big, you need to fit at least the motor and laser mechanism into something remotely portable.
2nd, its transfering at 8.5x regular MD, and data density is 4.6x times, so that can only mean more power, less battery life.
3rd, you have to have mp3 and atrac decoder, which may or may not be on the same chip, and even if its the same chip, it'll still be bigger.
4th, encoder for mp3.
5th, you'll need to have widespread use of MD data drive, otherwise people won't buy it because they can't put their mp3 on it (unless you have a computer interface, which probably would be firewire to make it small, and you'll need space for THAT, and the interface logic...)
Result: I'm envisioning something bigger than a MD and mp3 player combined.