I've made my own CompactFlash adapter by reusing a GBA cartridge (F-ZERO) and a CF connector of an old mainboard. Cut down the connector pins, prepare one side with wires and solder all pins to be grounded together onto the GBA edge connector. Connectors glued into the cartridge case and then started wiring the rest. It's a bit crampy and the cartridge doesn't go in the GBA slot all the way but it's barely visisble. Then I took some old Flash Advance Linker sourcecode and modified it for testing some IDE commands. So far, I could read a sector with it.
Anyway, it looks nicer and cheaper than getting a movie player cartridge. I hope I can use it with new DS firmware/software.