I did this from a USB stick -- it might be even less complicated from CD (I believe if you hold down the "C" key while booting, the Mac will boot from CD.)

Download 32-bit Linux Mint ISO image. http://www.linuxmint.com/download.php.

While the download is going, install rEFIt.  Test it.

When download finished, follow the steps in this forum post to make a USB stick that rEFIt will boot.  It was a little scary downloading a random tarball from a forum and using it to patch the ISO you're using to install a new operating system (rootkit anyone?)  But, after glancing at the files inside, I did it.  Looks like a lot comes from the syslinux package and stuff, which I've already trusted, and I guess I can trust this guy in the forum not to mess with it.  Follow the instructions, use rEFIt to boot off the USB stick.  For me rEFIt identified the USB stick as a legacy OS on a hard drive, but it booted OK.  Installation process goes like how you usually do a dual-boot install (if you want to keep Mac OS, that is.)

For me, GRUB failed to install to a partition (as opposed to MBR, the usual way to intall GRUB), crashing the installer.  I manually did grub-install with the --force flag, which causes it to use an unreliable blocklist method to install to the partition.  Even with this, I originally got an error something like "cannot read /grub/core.img"  I found somebody in a bug report suggesting to use ext3 on the boot partition instead of ext2, and it worked for me.  I suppose the blocklist may break if files moves around, but it's working for now.