Apple has opened up the ability to virtualize Mac OSX server as long as it is ran on Apple hardware. Previously they did not allow this at all, but times have changed and they now allow it to a certain level. Please see the following posts for more information:
To be honest, OS X is a sweet desktop environment. With all the usual UNIX-y tools available without resorting to things like CygWin. As a headless O/S, it's kind of crap. There's a grey area where as a developer one might want to have a stack of virtualized OS X systems to test different versions. Beyond that it's not really a good virtualizable platform. Apple allows it sufficiently for developers and test purpose, but there's no ESX-like virtual data center setup... And I can't figure out why anyone would want to.
While running OS X on anything other than Apple Hardware is a violation of the license agreement, and therefore you will be out of support, and could have legal action taken (IANAL, but the agreements strike as 'binding'), you could look at the opensource Darwin project, depending on what you want to "tinker" with.
Since the only way to legally obtain a license for OSX is to buy an Apple manufactured machine it isillegal to virtualise OSX, even if you purchase a "boxed" version, which in itself only contains upgrade media. For the benefit of forbes, the law is broken when breaching an EULA because the agreement is a contract. The contract is broken when a licensor attempts to install OSX inside a virtual machine or on non Apple manufactured hardware. This constitutes a breach of contract under civil law, irrespective of whether original install media is used, which is illegal. The sale of the boxed version is done in good faith and in itself for upgrade only. This is implicit because the media can only be installed on pre-licensed Apple manufactured machines. "Please don't use terminolgy [sic] regarding legalities if you are unfamilliar [sic] with them."
There is a HUGE difference between breaking an agreement by installing legally owned software on something Apple says you can't (civil tort) and doing something illegal like pirating software for financial gain (criminal offense). Please don't use terminolgy regarding legalities if you are unfamilliar with them.