Quo Computers' liquid-cooled Core i7 maxQ2 runs OS X, Linux, Windows 7 ... until Apple says otherwise
In theory, there's no reason you can't run Apple's OS X on non-Apple hardware, since Macs are using the same Intel processors as many PCs do. But actually finding someone who will sell you a computer running OS X who doesn't have Steve Jobs for a boss is quite a challenge, in no small part thanks to Apple's zealous legal team.