The new tiny Game Boy Micro that Nintendo announced at E3 2005 might come bundled with Play-Yan mp3 and mp4 extension.
This information is from an interesting interview at Gamespot with Nintendo Vice President George Harrison.
Play-Yan is a module that enables Nintendo DS and Game Boy Advance SP to play MP3 music and MP4 video files stored on an SD-Card. Play-Yan is only available in Japan right now. Play-Asia exports the Play-Yan module for $69.90.
IGN reviewed the Play-Yan MP3 player. Looks like Sony hit with the PSP into the right direction making it a fully integrated MP3 and movie player from the beginning.