The project was originally bound for the GBA, but RavenWorks eventually migrated his code and rewrote the graphics engine to take advantage of Nintendo's new hardware. Current plans for the touchscreen include a map display and a "tap-to-switch-weapons interface" (accompanying L/R weapon switching).
Cave Story: DS's newest demo allows you to explore the game's levels and mess with NPCs, its memorable soundtrack playing all the while. Though scripting, weapons, and many other features haven't been included yet, we're just happy to see that progress is still being made on the port. Make sure to download the demo and let RavenWorks know that his efforts are appreciated!
If you don't have the homebrew tools necessary to test this early build, we have a few screenshots for you to preview in our gallery. You can also disguise yourself with a Mimiga mask and sneak past the post break for some shakycam video we dug up.
Gallery: Cave Story DS