So you figure you will use launchd since it is supposed to be some sort of replacement for cron on OS X eh?
Well if your machine sleeps (at least under 10.4), the timer for launchd sleeps with it (yes, you heard me right, it seems to be relative to the sleep / wake time on the machine).
Probably not so bad for most tasks, but if you want to run an overnight task say, I don't know, overnight, you can't count on it running overnight if the machine went to sleep! It will shift the start time of your launchd request by the amount of time the machine was asleep.... Ugh!