I think that the concern is that the "at" commands will not execute if the computer is asleep or off. I have not tested this, though.

-Mark

I think the concern is that atrun, which actually runs the at jobs, will hit the hard drive every so often (defaulting to five or ten minutes) to check for jobs. If a PowerBook user set their hard drive (but not the processor) to go to sleep after a while, it'd be spun right back up the next time at checked for jobs, regardless of whether there was a job to run or not.

If the whole computer is asleep, then cron jobs and launchd aren't running, so an at job won't wake a sleeping system up.

atd relies on cron to run its jobs. cron doesn't cause any "power management" issues because it preloads the table into memory and reloads after updates.

at is a cronjob, which by default is scheduled to run every minute. It scans its directory for jobs that have expired and runs them. With 1 disk hit per minute, the drive will not sleep much which causes problems, especially on laptops.