Hacked Metroid VideosDarkFader has been kind enough to document his hacking efforts with a short video. This video basically shows how he uses a gb flash cart to load code into the DS via the GBA port. From what I understand he boots from the DS card slot through the FPGA, this allows him to insert a command to look at the GBA port and execute some code from there. Right now he is able to execute ARM 7 code from the GBA port but still needs to boot via the DS card slot in order to execute ARM 9 code. It might be possible to execute ARM 9 code solely from the GBA port though no one is certain yet.
Quicktime video here. Original AVI file which might not play back without codecs here. If you are having problems playing the AVI download VLC