Natrium42, the man at the frontlines of DSLinux development, has recently created a blog. He is currently designing a simple and compact passthrough (called Passme) to be built with releatively easily accessible and low cost parts. For those who are new to the homebrew scene, in the DS's case, a passthrough is an apparatus that allows you to run your own code on the DS.
In his own words:
I have designed a small and simple passthrough using a CPLD (Complex Programmable Logic Device). Expected cost is $30 in parts. jzawaki is going to build and test it in about 2 weeks since this is how long it takes to receive a custom PCB.
Natrium42 includes schemactics and a partlist for his passthrough.
Rendered pictures of Passme:
I want to repeat, to prevent confusion, that this device is in the design phase and has not actually been built and tested yet. The plans look pretty sturdy though, in my opinion.