On Linux and (I believe) FreeBSD, the ability to set processes to different levels of realtime priority has been in place for years. Very recent (last couple weeks?) versions of the new Linux O(1) scheduler can do "idle" priority, and it will probably be in the 2.6 mainstream kernel.
Implementing the idle priority is hard, because if an "idle" task gets a lock on a crucial resource and gets preempted, any non-idle tasks that need that lock must then wait for the system to become idle -- potentially an indefinite amount of time.
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Implementing the idle priority is hard, because if an "idle" task gets a lock on a crucial resource and gets preempted, any non-idle tasks that need that lock must then wait for the system to become idle -- potentially an indefinite amount of time.