Scheduling tasks in Leopard
Apparently you cannot schedule task in OS X on your terms. The only option you have is to put the script in the periodic.daily (or weekly, monthly) directory.
For reasons known only to God and some morons at Apple, they decided to remove crond. I just spent a hour at the local Apple store with there techs.... no dice. It is either in the default periodic directories of which you have no control of the actual times, or not at all.
Good job Apple! If you are trying to dumb down your OS to a Windows level; your on the right track.