Hakentoshes violate Apple's EULA for Mac OS X. Whether EULA violations are illegal is a subject of contention. There have been no decisive legal tests (to my knowledge). Violation of the EULA definitely frees Apple of any warrantee or support obligations.
Even when Apple went after Psystar they did not contest them based on the EULA as everyone, including Psystar, expected. Instead they won based on violations of copyright and the Digital Millennium Copyright Act.
Psystar Corporation - Wikipedia, the free encyclopedia
Apple has no history of attacking, or even commenting on individuals who choose to install OS X on personal computers, so long as they are not using illegal/pirated copies of OS X. Companies that do this in an attempt to make a profit are another matter. Apple sees them as both competition and a threat to Apple's brand name, and squashes them ruthlessly.
This is understandable if you remember that Apple almost bled to death when they allowed clones. Even though they received licensing fees, it wasn't enough to compensate for the loss of sales to cheaper alternatives. It left scars on the company psych, I think. Some fans seem to feel this way too.
Of course, it didn't help that without Steve at the helm they were not producing anything better than blah designs and ideas themselves.
Ok, lecture mode off.
The Hackentosh is a sort of gray area that no one cares about, as long as it is primarily populated by hobbyists.