It is possible to have a dual boot system with MacOS and some other UNIX system.
I had an older iMac with a single hard drive partitioned in a way that allowsed to have both Mac and UNIX running. It was/is running MacOS9 next to some UNIXes: I had OpenBSD on it, later on Yellow Dog Linux, and finally Fedora Core, which is still installed.
Most difficult to install was OpenBSD, because even the README files required technical expertise to figure out what had to be done. With Yellow Dog success was depending on the version - older versions did work better on it(strange!). Fedora installation was easy, full GUI interface for the install process.
Haven't used that iMac for years. Current distros should work even better, I guess.
Xen in Hardy