Previously the ability to do this has been available to Mac OS X Server only. The End User Licensing Agreement that is included with Mac OS X Lion GM Seed reveals that you are now permitted to run up to two additional instances of Lion on your computer.
From the 10.7 EULA:
(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.
Users who are planning to do this will be happy to know that extra licenses are not required.