Copyright act section 117:
(a) Making of Additional Copy or Adaptation by Owner of Copy. ? Notwithstanding the provisions of section 106, it is not an infringement for the owner of a copy of a computer program to make or authorize the making of another copy or adaptation of that computer program provided:
(1) that such a new copy or adaptation is created as an essential step in the utilization of the computer program in conjunction with a machine and that it is used in no other manner, or
(2) that such new copy or adaptation is for archival purposes only and that all archival copies are destroyed in the event that continued possession of the computer program should cease to be rightful.
(b) Lease, Sale, or Other Transfer of Additional Copy or Adaptation. ? Any exact copies prepared in accordance with the provisions of this section may be leased, sold, or otherwise transferred, along with the copy from which such copies were prepared, only as part of the lease, sale, or other transfer of all rights in the program. Adaptations so prepared may be transferred only with the authorization of the copyright owner.
So in other words, it's legal to hack OS X to run on a PC, but it's not legal to resell a pre-hacked version of OS X. Although it would be legal to sell the hacking tools to let you do it yourself. Psystar is actually in the wrong here, but not for the reasons Apple says.
DMCA: "No person shall circumvent a technological measure that effectively controls access to a work"
The encryption in OS X does not control access to OS X, it rather controls OS X's access to hardware. It's like the difference between DRM in an mp3 that only enables it to play on certain devices regardless of how much it's been redistributed, to DRM that restricts playback and copying to a single persons account.