set theLoad to do shell script "uptime | sed 's/.* //g'"Save the script, make it executable, and then use 'cron' to schedule it to run at regular intervals, and your Mac will put itself to sleep when the CPU load has been below your threshhold.
if (theLoad as number < 0.8)
tell application "Finder"
sleep
end tell
end if
The forum thread has more info on the cron task if you'd like to see how that step is done...
[Editor's note: Script changed to reflect comments posted below - thanks all!]

