Mac OS X SLA (located at http://www.apple.com/legal/sla/macosx.html
), section 2.A:
A. This License allows you to install and use one copy of the Apple Software on a single Apple-labeled computer at a time. This License does not allow the Apple Software to exist on more than one computer at a time,and you may not make the Apple Software available over a network where it could be used by multiple computers at the same time. You may make one copy of the Apple Software (excluding the Boot ROM code) in machine-readable form for backup purposes only; provided that the backup copy must include all copyright or other proprietary notices contained on the original.
If my thinking is correct, it would be a violation of the agreement to install Mac OS X on a completely custom-built machine (or even an old one ordered from, say, Dell)?