UPDATE:
VMware said that this should not be the case, the next update will check the version of Mac OS X before installation. Only OS X Lion and Mac OS X Server are allowed for virtualization.
From MacNews:
One thing that was revealed is that you could run earlier versions of Mac OS X as a client under 4.1 (this was not previously possible with version 4.0.1). Given the licensing, we were incredibly surprised (although delighted) to see this. Unfortunately, while incredibly useful for developers wanting to test on multiple versions, this turns out to be an oversight.
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VMware has just updated VMware Fusion 4.1 with added support for installing Mac OS X Leopard and Snow Leopard client in addition to OS X Lion as noted by many including The Mac Observer.
This is certainly a good news for many Mac users, especially the ones who really need Snow Leopard and Rosetta support. Now you can have iCloud and Rosetta support in one Mac.