I have posted versions of the ChibiOS/RT RTOS for AVR Arduinos, Due, and Teensy 3.0 as ChibiOSBeta20121212.zip http://code.google.com/p/beta-lib/downloads/list.
A small patch is required to run ChibiOS on Due. This patch involves a hook for access to the Cortex M SysTick interrupt. Future versions of the Arduino IDE should have this hook. I included the patched version of wiring.c for 1.5.1r2.
I have packaged ChibiOS as two libraries, ChibiOS_AVR and ChibiOS_ARM. It would have been possible to combine these into one library but that would require a lot of modification to combine of the original ChibiOS files with conditional compilation.
A number of examples are included.
ChibiOS is extremely fast and small on Due. One example measures the time for a semaphore and a context switch using a scope. I use digitalWrite to generate pulses for this test. The time to execute digitalWrite is almost the same as the total time for giving a semaphore and doing a context switch between two threads.
I plan to post FreeRTOS soon. I want to update the AVR version of FreeRTOS and do more testing first.