Virtualizing the desktop versions of Leopard and Snow Leopard goes against Apple’s licensing terms, despite the fact that OS X Lion’s license allows for multiple virtualized installs.
VMWare’s official blog post reads as follows:
VMware Fusion 4.1 was released late last week and includes many great improvements. One change was the introduction of a new license verification step for users to verify they are in compliance with the OS licensing terms.
When the license verification step was added in VMware Fusion 4.1 the server edition check was omitted. We are preparing an update.
If you want to be able to use Leopard or Snow Leopard under Lion (such as for Rosetta support), it’s recommended that you don’t install this patch, although future patches will contain security and functional updates that will be hard to resist.