I've done it multiple times. At the moment I'm on a MacBook which can dual-boot between OS X and Debian Testing. It works fairly well, although it is a little bit of a pain to set up, compared to regular PCs. I recommend special instructions, such as the ones posted here:

• http://www.mactel-linux.org/wiki/HOWTO - a general wiki relating to hacks and how-tos and installation
• http://wiki.debian.org/MacBook - an excellent guide for installing Debian Testing on a MacBook
• http://desrt.mcmaster.ca/macbook.xhtml - little hacks for Linux annoyances specific to Macs
• http://gentoo-wiki.com/HARDWARE_Apple_MacBook - guide for installing Gentoo on a MacBook, especially good is the kernel compilation (any distro)

Major annoyances at first for me were: the trackpad (only one button, and no 2-finger scrolling), display wouldn't go into full 1280x800 mode (used a stretched 1024x768 mode), and the fact that the thing wouldn't sleep when the lid was closed.

Although I still haven't solved a few of them (mostly I don't have enough time to tinker with it), the fixes for thesee issues are described in the articles already mentioned. Definitely check them out to help make Linux as functional as Mac OS X.

And make sure you backup your hard drive before trying any fancy stuff with Linux -- more than once a Linux distro has wiped out my partition table.