On Thursday VMware released Fusion 4.1, an update to its virtualisation software that adds numerous small improvements, and a huge unheralded change that could provide Mac users with much more flexibility when it comes to running old versions of Mac OS X.

The features documented in the Fusion 4.1 release notes include support for full screen mode on Lion (including a "Smart Full Screen" mode that doesn't leave users of multiple display Macs out in the cold), performance and graphics improvements, and support for Lion features such as FileVault 2 and Lion Recovery.

But one big change with this update isn't documented anywhere: The software has been modified so that it will run the non-server versions of Snow Leopard (Mac OS X 10.6) and Leopard (Mac OS X 10.5). Previously, VMware Fusion supported virtual Macs running Lion, Lion Server, Snow Leopard Server and Leopard Server.

Apple's operating-system license policy during the Leopard and Snow Leopard eras specified that only the server version of Mac OS X could be run in a virtual machine, and then only on Apple hardware. When Lion was released, that policy changed to allow both Lion Server and the non-server version of Lion to be virtualised.

When a user tries to install Leopard or Snow Leopard in Fusion 4.1, a dialog box appears that says, "Verify that the operating system is licensed to run in a virtual machine." In essence, this removes VMware from the position of having to evaluate and enforce Apple's operating-system licence, and instead leaves the decision in the hands of users.

As a VMware tech note explains:

VMware Fusion 4.1 changes the behaviour of the new virtual machine assistant when creating a Mac OS X virtual machine. Starting with Fusion 4.1, you are presented with an additional prompt to confirm that the operating system is licensed to run in a virtual machine. This additional prompt reminds you that installing Mac OS X in a virtual machine is subject to the licence agreement that accompanies the Mac OS X software.

VMware recommends consulting the licence agreement accompanying your Mac OS X software for the terms and conditions that apply.

If you confirm compliance with the applicable licenses, the assistant proceeds to the next step. This behavior is identical for the Leopard, Snow Leopard, and Lion releases of Mac OS X and their variants.

A cursory glance at the Snow Leopard licence specifies that it can only be run on Apple-branded hardware, but doesn't seem to specify anything about running in a virtual machine, though the single-use licence specifies running "one copy... on a single Apple-branded computer".

The Lion licence specifically mentions allowing "virtual operating system environments on each Mac Computer".