Call me paranoid, but I try to keep my iBook as secure as possible, so if it's ever lost or stolen, the culprit will have a lot of hard work to do before getting to my files.
One of the things I've done is set up the Open Firmware password to prevent the machine from being booted from anything but its internal hard drive. It also prevents the machine from going into Target Disk mode, and it also prevent booting to single user mode (command-S at startup). The machine will always boot to the login window.
This presented a problem for a while, because I couldn't figure out how to fsck my disk when I suspected problems. But there is a way ... rather than booting directly into single user mode, we're going to go further than that and then go backwards to it. It's like going two steps forward and one step back. Here's how:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020725085134490