The iPlayer Team has released a device that will enable the playing of all sorts of un-converted video and music on your DS. Ingeniously, it's called the DS iPlayer!
Here's the (badly worded) spiel from team: "iPlayer is a media player card for Nintendo DS users, which is designed with a high-performance decoder chip inside, allowing hardware decoding, will be able to play the main format of videos directly without converting.
"After more than one year hard working, iplayer Team finally complete the product development, considering as a milestone, because of the realization of playing AVI/RMVB on DS directly by iPlayer, which is also the unique one could to do so world-wide. iPlay is able to play all the main format of videos such as RMVB, RM, AVI, FLV, MPG, WMV on DS, and no converting, allowing you watching movies and listening to music on DS more convenient."
More information over here