It's MP4 and it's a standard. You can play un-DRMed MP4s (like the ones you make yourself, for example) on many different players.
And you're funny. Just as MD is finally dying, you're leaping to it. At least the discs will be cheap, I suppose.
(I've got a 20GB 4th gen iPod colour, and wish I had a 60GB - changing discs is so last century)