I would like to run older versions of OSX on my Mac Mini. Apple dropped support for Classic Mode and for PowerPC code, which broke SoundDiver, an important tool in my home music studio. All I've been able to dig up so far about OSX virtualization is that it used to require OSX Server but won't with OSX 10.7 Lion.
SoundDiver 3.x still works just fine with Windows XP. It stinks that Apple discontinued it for OSX after treating it like an unwanted stepchild for years.