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 , and is available from PPC distribution ftp sites (e.g. ftp://ftp.linuxppc.com/contrib/software/Utilities/System/). There is also an older utility called snooze available from the same sites that just puts the PowerBook to sleep.
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.