[mapgrep]

You are correct, the OS X EULA since 10.7 Lion has allowed two additional instances to be run within virtualization at no additional charge. Here is the EULA for 10.8, search for "virtual": http://www.apple.com/legal/sla/docs/OSX1082.pdf

("...you are granted a limited, non-transferable, non-exclusive 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...")