Hmm... THis one bears notability, as the actual Apple EULA states this:
2. Permitted License Uses and Restrictions.
A. Single Use. This License allows you to install, use and run one (1) copy of the Apple Software on a single Apple-labeled computer at a time. You agree not to install, use or run the Apple Software on any non-Apple-labeled computer, or to enable others to do so. 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.
B. Family Pack. If you have purchased a Mac OS X Family Pack, this License allows you to install and use one (1) copy of the Apple Software on up to a maximum of five (5) Apple-labeled computers at a time as long as those computers are located in the same household and used by persons who occupy that same household. By “household” we mean a person or persons who share the same housing unit such as a home, apartment, mobile home or condominium, but shall also extend to student members who are primary residents of that household but residing at a separate on-campus location. The Family Pack License does not extend to business or commercial users.
C. You may make one copy of the Apple Software (excluding the Boot ROM code and other Apple firmware that is embedded or otherwise contained in Apple-labeled hardware) 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. Apple Boot ROM code and firmware is provided only for use on Apple-labeled hardware and you may not copy, modify or redistribute the Apple Boot ROM code or firmware, or any portions thereof.
Replacing, or upgrading the logic board, in an Apple-labeled computer, such as a Powermac G5 is technically within the terms of the EULA, even if you do not use an original Apple mainboard. The EULA does not specify what specific hardware must be used to run OSX.
The license itself clearly makes the distinction between an 'Apple-labeled computer' and 'Apple-labeled hardware'. You could technically further break this restriction by labelling any system with an Apple logo, made by Apple. (Apple used to supply those labels freely.)
The definition of 'Apple-labeled' is not clarified in the EULA.
Only the boot-rom information, or other 'embedded firmware' is restricted to non-Apple hardware, and as a 'Hackintosh' made from an Apple system, uses a custom bootloader for the firmware on your mainboard, and not the Apple firmware, this clause would not restrict you from using a mainboard inside an Apple system, other than one made by Apple, to run OSX.
From 10.5 onward (possibly late versions of 10.4), the EULA does not exclude virtualisation. You could run Linux on a Mac, and OSX 10.5 on Linux (in a VM); or OSX (any version) on a Mac, and another licensed copy of OSX (10.5 or later, and possibly 10.4) in a VM, entirely legally.
On a side-note, some countries will entirely reject the enforceability of this kind of EULA.