crontab itself doesn't need to be turned on as such—it's used only to tell cron to use a particular crontab file. However, cron does need to be running in order to know to do anything. I don't know about Leopard, but, on Tiger, cron is started up by launchd in response to a file called com.vix.cron.plist in /System/Library/LaunchDaemons. (That file records the path, tells it to RunAtLoad, and gives it WatchPaths and a QueueDirectory, which are /etc/crontab and /var/cron/tab, respectively.)