The OS X Mavericks EULA grants the rights
B. Mac App Store License
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using OS X Server; or (d) personal, non-commercial use.
By my reading, this implies that I can't use VMWare ESX as the base-level OS hosting OS X guests, as then it wouldn't be "already running the Apple Software". However, VMWare definitely support running OS X as a guest under VMWare ESX. Presumably VMWare wouldn't support something that violates the OS X EULA, or they would have continued the support for OS X guests running under VMware workstation on windows that they had for a short time.
How does this add up?
Also, If we assume that it is OK, what happens if I want to run 3 or 4 guest copies of OS X on my mac mini? The EULA implies that it's not, however the EULA also contains this clause
Volume or Maintenance License.
If you obtained the Apple Software under a volume or maintenance license program with Apple, the terms of your volume or maintenance license will determine the number of copies of the Apple Software you are permitted to download, install, use and run on Apple-branded computers you own or control. Except as agreed to in writing by Apple, all other terms and conditions of this License shall apply to your use of the Apple Software obtained under a volume or maintenance license
This seems like it would provide a solution to both of the above, but I can't figure out if it's possible to purchase a "Volume or Maintenance License" for OS X, as nothing I've been able to find on their website has any information related to it.