Lately I've been working to make a spinoff from my YoutubeDS project from a couple of years ago, to make it into a useable video player. After a lot of bugfixes and improvements I've managed to get pretty nice results. Mainly on DSi it works very well. It plays mpeg4 video and aac audio in a mp4 container. Note that the code is pretty shitty as it is a modified version of youtube ds. This project might be incorporated in some general purpose media player for ds(i) at some point, but concrete...
MPEG4 Player for DS and DSi
MPEG4 Player for DS and DSi