Many people have made it clear that there is nothing technically stopping OSX from running inside of ESX or Workstation. At this point, as mittell said, it lies in the licensing from Apple and how it is worded. As a result, the ability to run OSX is blocked. There are ways to hack in Darwin and some people have hacked OSX enough to get it to run, but there is no supported or legal way currently.
Thomas H. Bryant III
