If you're an old UNIX die-hard using Leopard (and I think Tiger as well) you may have noticed that at doesn't seem to work. That's because launchd (How do I hate thee? Let me count the ways…) has subsumed atd as well as all the other useful scheduling things like cron and friends. at's manpage doesn't tell you there is no atd, and implies that cron calls atrun every five minutes (incidentally, atrun's manpage says every ten minutes). Apparently in Tiger the manpage was more helpful, telling you how to enable at jobs with launchd, but no more.
If you do want to enable at functionality, this is the trick:
launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist
Be warned that it's apparently disabled by default due to power management concerns, so it might not be laptop-friendly. I'm going to see if it makes a noticeable difference to my laptop usage and/or battery life over the next few days and if it does I'll blog it.
Launchd runs atrun every 30 seconds. This is configurable by hacking the plist file mentioned above.