The R-Unit will be a device that will plug into the cartridge slot on a Gameboy Advance (including the new SP), and play mp3 files stored on a removable medium (currently CompactFlash).
It certainly isn't at that point yet. I don't know enough about CompactFlash integration to bother trying to make hardware. Perhaps you do? If you'd like to give me a hand (even if it's just advice), drop me a line. You can get my contact info from the Overview page.
I am writing the software in C, and using DevKitAdv to compile it. To test the software, I am using a variety of emulators, as well testing on actual hardware using Visoly's Flash Advance Xtreme.
To decode the mp3s, I plan to use mpglib (LGPL), from Michael Hipp (maker of mpg123).
All of the software I write will be released under the GNU GPL, and the hardware designs will also be free (I'll figure out some sort of GPL-like license for that when I have some hardware to license : ). The software may include some components that are not full GPL, but I will try to use only software that has some sort of accepted license accepted to be truly free.
I've been made aware of a possible copyright infringement, as the GBA hardware will only run a ROM image if it has a copy of the Nintendo logo that exactly matches its own. At first glance, it would seem like copyright infringement to copy this logo for use in an unlicensed program, but an article about the court case Sega vs. Accolade says this is simply reverse-engineering, and protected under Fair Use rights.