I spent this past weekend porting Repetier-Firmware to Printrboard and Teensyduino. It took me a while to get it working properly, because the code intermixes calls to fastio and Arduino, and the pin definitions in fastio.h were different from the Teensyduino Arduino library. It is now working pretty well, so Printrboard and Teensylu users now have yet another option for RepRap firmware. I’ve already submitted my changes to Repetier, and he’s merged them into his code. Download it here: https://github.com/repetier/Repetier-Firmware.
In case you haven’t tried it, Repetier also has an interesting host program, Repetier-Host, which works with other firmwares as well as Repetier-Firmware.