From Lifehacker:

Just like on Windows, there are quite a few ways to break into a Mac, but many of them are variations on the same thing, so we're going to highlight the two easiest ways—one with a Mac OS X installer CD and one without—and show you how to keep yourself protected. Note that while these two methods will get you into the OS without knowing the password, you can always just use our previously mentioned "lazy method" with a Mac too—just boot up the computer with a Linux Live CD and start grabbing files.

Read the full article: http://lifehacker.com/5681710/how-to-break-into-a-mac-and-prevent-it-from-happening-to-you

guess there is still a difference between providing a Windows Driven nutrient medium for anything higer leveled IQ then a piece of wood to break in, and the physically security of a PC itself (no matter what OS is on it)

the noted security riscs also aply for Windows, unix and Beos, dos, macos, os/2, cp/m Driven Workstation and server Computers.

as the golden datacenter rule: a Computer is that Secure as its discs are

If you use Firmware Password Utility, that blocks any kind of startup of a Mac in any mode. No CD/DVD start, no Target mode, nothing without the password.

There is a way to break that, but one must be pretty much a Mac mechanic to do it.