I recently purchased a 12in Powerbook G4. Being an AO member and a security nut, the first thing I did was lock down the firmware and the factory install of OSX. Unfortunately there wasn't much more I could do to lock it down. I was realy dissapointed. The fact is, the PB is the most secure laptop I have ever, ever seen, or heard. Here are some of the highlights->

--Firmware retains state un-powered, so you cant just pull the battery to reset it like you can do in a PC.
--Command security mode asks for password on firmware commands and target disc mode.
--Don't even think about single-user mode.
--Unix file permissions.
-- BSD
--OSX is really secure by default.
--OSX's IP firewall is impeccable.
--Disk Copy can create AES encrypted dmg's
--Root is disabled by default

In the end you have probably the most user-friendly and secure end-user OS in the world running on top of hardened firmware. Wow, that baby is rock solid.