Can Mountain Lion be used as a guest OS under 10.7 Lion?
As far as the license says, no.

You have the right to:

(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.


So, as far as the license goes, you must already be running Mountain Lion in order to virtualize it.