When Apple’s new Lion OS X releases later this month a new feature within the OS X client version will now allow multiple virtual copies to run on the same machine. A feature that was only previously enabled on the Mac OS X Server software edition.
The new EULA for Lion OS X 10.7 will now enable users to be able to run one or two virtual Mac instances on each physical Mac using already existing Virtualisation software such as VirtualBox, Parallels and Fusion.
The new feature will be great for developers now allowing them to run multiple Lion OS X environments for testing their applications and creations. The change in policy by Apple indicates how close Lion OS X and its OS X Server side add-on will be, when Lion OS X Server becomes an add-on pack available from the Apple Mac Store on release.
The new Golden Master version of OS X Lion 10.7 now includes the feature if you cant wait until its released next week.
Source : Mac Rumours