With the install of a couple of tools, MacFUSE and fuse-ext2, you can mount ext2/ext3/ext4 filesystems in OS X. First caveat: I haven't had the guts to write to my Linux partition from OS X. I'm not sure how stable it is and I had no pressing need to experiment. But feel free to be your own guinea pig! That being said, I've had no problems mounting as read-only and copying files from Linux to OS X. So here's a quick how-to.
After you've installed MacFUSE and fuse-ext2, reboot unless you're one of those smartypants who knows you don't have to. I didn't want to find out if I was wrong, so I wimped out and rebooted. This is already going off on a tangeant. I'm also being distracted by pianobar running on Tiger (more on that later). I'll try to be better.
Anyway, once all that's done, fire up Terminal.app and create a new directory in your /Volumes folder with this command:
(UPDATE: For some reason the folder disappears on me after reboot. I can just create it again as needed, but I don't know why it does this.)
I named mine /Volumes/Linux, but you can choose your own. Then you want to find out the identifier of your Linux partition. So run
diskutil listand you should see your Linux partition among the output. The right column gives the identifier, in my case disk0s5. Now with that handy info you can mount your Linux partition with:
sudo fuse-ext2 /dev/disk0s5 /Volumes/Linux
Obviously replace disk0s5 and Linux with your own particulars. Only there's a bit of a problem here. In Tiger, and I think this is only an issue in Tiger, when you open /Volumes/Linux in the Finder, the folder disappears. Rather vicious bug, but you can still access it in the Terminal with standard commands like ls, cd, and cp, etc.
Also, the above mount command is read-only. If you want to try read/write and see how that goes, add "-o force" to the command.
Unmounting is a simple
sudo umount /Volumes/Linux.
If you want the reverse, say total access to your OS X volume while booted in Linux, that involves a couple of issues. First, it'll be read-only unless you disable journaling in OS X. You also won't have access to OS X's home folder (except ~/Public) due to permissions issues. You can read all about how to get around that here and here. I haven't gotten around to experiencing this firsthand, so I'll farm it out to those links. Have fun!