According to a Lifehacker how-to, it is possible to dual-boot an Intel-based Mac with OSX and GNU-Linux, but you'll need to shrink your HFS partition and create an EXT3/4 partition and a swap partition in that space (instead of installing in/on an HFS partition). The following is verbatim from that How-To:
Boot your Mac into OS X. If you're lucky, this may be one of the last times you have to. First, install the rEFIt boot manager. It's a straight-forward installation—just download the disk image and double-click the installer. To confirm that the app is working, reboot your system.
Next, make space for your [GNU/Linux] installation. Now you'll need to decide how much space you want to give your new Ubuntu installation — I gave it about 40GB — plenty of breathing room and way more than basic system requirements, but if you want to use [GNU/Linux] full time, give it as much space as you can afford.
[In OS X,] Open Disk Utility (Applications > Utilities > Disk Utility.) Select your hard drive from the list on the left, and click the Partition tab on the right. You'll see the current partition layout. Click the right corner of the current partition and shrink it to the size you want. The display will show you the minimum size, so don't worry about going too far. Alternatively, just select the current partition and type in the final size (total hard drive space - amount you want [GNU/Linux] to have) in the Size field on the right.
Click apply. Disk Utility will shrink the current partition for you and free up space for your [Linux] install.
Pop in your freshly burned [GNU/Linux] CD and reboot. rEFIt will appear and ask you if you'd like to boot to the CD. (Beats holding down the C key.) Select the CD and let [GNU/Linux] start up. [Follow the normal installation procedure as you wouldfor a PC.]