It should not be that much of a trouble. I've done it the other way (running OSX on a custom PC) but that's a different story. There's some guides around on how to manage this, here's one for you to check: https://help.ubuntu.com/community/Intel_iMac
Apple uses "normal" hardware in thiere machines nowdays. Intel CPU, Realtek soundchip and Nvidia GFX I think. I actually run OSX with drivers gathered from Debian.
Is there a special reason for this change? I think OSX is kinda neat
you can get Macports for OSX that will let you install a whole lot of Linux-packages and you can install OSX with a custom kernel. It's called Vanilla kernel and is just more like the one used with most Linux dists. With this kernel you could for example install KDE onto OSX but I'd guess that's kinda messy.
edit: The rEFIt tool is a that will install a bootloader on the hidden EFI partition that is used by OSX. If you don't plan to use OSX, than use another bootloader.