Mac OS X 10.5 Leopard’s license allows it to be virtualised, provided that
- it is virtualised on Apple hardware,
- sufficient licenses are purchased to cover each virtual instance
2. Permitted License Uses and Restrictions.
A. Mac OS X Server Software. This License allows you to install and use one copy of the Mac OS X Server software (the â€œMac OS X Server Softwareâ€) on a single Apple-labeled computer. You may also install and use other copies of Mac OS X Server Software on the same Apple-labeled computer, provided that you acquire an individual and valid license from Apple for each of these other copies of Mac OS X Server Software.
via comments on the MacEnterprise mailing list.