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 <stephan at jvc.nl>, and is available from PPC distribution FTP sites. There is also an older utility called snooze available from the same sites that just puts the PowerBook to sleep.