PowerBooks don't support the APM specification, but they have a separate protocol for their PMU (Power Management Unit). There is a free (GPL) daemon called pmud that handles power management; it can monitor the battery level, put the machine to sleep, and set different levels of power consumption. It was written by Stephan Leemburg. There is also an older utility called snooze available from the same sites that just puts the PowerBook to sleep. PBButtons now includes the functionality of pmud.
Cron works fine on my laptop as I never shut it off completely. I only put it to sleep. When it wakes up, the unexecuted cron jobs from the sleep period all run.
This part is a courtesy of Steven G. Johnson.