Nintendo DS MP3 player will be made available in Europe for the small price of 29.99 EUR. No release date has been given.
The Add-on slots into the Gameboy Advance port on the Nintendo DS. This device will also be playable on the Gameboy Advance itself, so it's accessible to a wide audience. To store the actual music, you'll need SD cards. Up to 2GB size cards are supported and this will allow you to store about 500 or so songs. You will also have the lovely option to change skins, like you can with programs such as Winamp. Pretty neat stuff.