While the people over at gbadev.org are busy trying to wirelessly hack the DS, there are a couple of people making real progress with the DS game carts themselves. Their target is not the actual main chip itself because that is read only, but instead the EEPROM which is used for game saves. EEPROM -unlike the main chip which is MASK ROM- can be erased and reprogrammed. By acessing the flash memory there is a possibility to overflow the buffer and get the Processor to execute some desired code, where for example it could load a linux distro from a flashable gba cart.
Over at the dslinux forums there is a juicy thread
where ChaosKnight has dismantled his Metroid Prime : Hunters card and is trying to make it mount by hooking it up to a usb thumbdrive.
ChaosKnight also points out work by DarkFader
that seems to also be making progress. There is a lot of juicy data on his site including code examples and dumps. Here is DarkFader's speculative pinout mapping.