Leopard Server License allows virtualisation on Apple Hardware

Mac OS X 10.5 Leopard’s license allows it to be virtualised, provided that

1. it is virtualised on Apple hardware,
2. 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 “Mac OS X Server Software”) 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.