I was frustrated about cron not running because my Mac was asleep late at night so this weekend I wrote a couple of tools to make sure that cron was "on the job" at critical times.
The first is dailyWakeup, a command line program that uses the Power Manager API to schedule a wakeup of the Mac a little before the time for cron to do its work. I also have cron run dailyWakeup to queue the request for the next day. This is probably not for newbies, since the program has to be customized and recompiled for each installation.
The second tool is ShowPMQueue; this program displays all of the events in the Power Manager queue. If the actual executable program is suid'ed to root, then events can be added or deleted from the event queue. This program (properly installed) make it very easy to schedule wakeups, sleeps, boot times and power down times.
Source and a meager discussion are located here:
http://homepage.mac.com/levanj/Cocoa
[robg adds: I haven't tested these scripts, and they are clearly not targeted at newcomers to the UNIX side of OS X...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040322235349936