Running Mac OS X in non-apple hardware
It is against the terms of the EULA:


  2. Permitted License Uses and Restrictions.
  A. Single Use License . Subject to the terms and conditions of this License, unless you have purchased a Family Pack or Upgrade license
  for the Apple Software, you are granted a limited non-exclusive
  license to install, use and run one (1) copy of the Apple Software on
  a single Apple-branded computer at a time. You agree not to install,
  use or run the Apple Software on any non-Apple-branded computer, or to
  enable others to do so.


However, Apple has historically not prosecuted individuals who violate this for hobby purposes, and have only gone after people who sell preinstalled systems (such as Psystar) or published how-to guides that involve downloading pirated/pre-modified versions.  And, of course you get absolutely no customer support or guarantee that it will continue to work.