There is a new media player, called the
[Only Registered Users Can See Links. Click Here To Register], being released July 1, 2009 for the DS. The iPlayer is a slot 1 flash card (the slot where the game card goes), and will support most popular audio and video formats, homebrew, and is skinnable. It's going to support 640x480 video format, no more converting. Just what I been waiting for. --Look to the bottom of the post for info about a GBA emulator that works on iPlayer.--

EDIT- Now the the iPlayer has been released and I see that it costs $45 USD, I'll wait for a price drop.
It seems a though darkchen has released a NDS-GBA emulator that works on the iPlayer, you need to have an iPlayer to install the GBA emulator to. The reason is that the iPlayer provides a little extra processing power, enough to get this emulator to function. I hate to copy and paste, so visit the original threads and GBAtemp and
[Only Registered Users Can See Links. Click Here To Register].