You can run Linux on a Mac as described on a wide variety of sites in a dual/single boot environment. My preference is running Linux guests in VMware Fusion or VirtualBox, for software testing purposes.
However, and I don't want to try and talk you out of this too much - nor do I want to incite an OS battle, I wouldn't bother.
I'm a long time Linux user (~>15 years), and considered doing this when I got a Macbook Pro from work.
However, Linux as a workstation OS platform typically means hardware hassles. Wifi and sound often mean times of lost productivity trying to get things to work properly. Not to mention that I have an iPhone, and syncing that with an OS that doesn't run iTunes natively means a headache waiting to happen. This doesn't take into consideration all the other software hassles that come with running Linux.
Mac OS X, of course, works perfectly with the hardware, since it was designed to do so. That is of course the primary benefit to buying a Mac, is the OS is tightly integrated. Also, because Mac OS X is a certified Unix, that means it can (and does) run all manners of Unix (and Linux) software. With the MacPorts project, a wealth of software is a
port install away.
OS X provides all the great things I like about running a Linux workstation (Unix/Unix-like utilities, powerful command-line and scripting ability, etc), and none of the things that have frustrated me over the years.