There's a free Technology Preview of Fusion 5 that's time-limited, but probably not so much that it wouldn't work until Fusion 5 ships. Mountain Lion is officially supported as a host and guest. I've been using it daily for a couple months to run Windows (7 and Server 2008 R2, plus the various previews of 8 and Server 2012), FreeBSD, and OS X (Lion, Snow Leopard Server, and Leopard Server), and it seems stable.
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I got bit by the same thing with VMware. You must be on Fusion 4 for Mountain Lion. Sucks because I had no plans on upgrading.
There's a free Technology Preview of Fusion 5 that's time-limited, but probably not so much that it wouldn't work until Fusion 5 ships. Mountain Lion is officially supported as a host and guest. I've been using it daily for a couple months to run Windows (7 and Server 2008 R2, plus the various previews of 8 and Server 2012), FreeBSD, and OS X (Lion, Snow Leopard Server, and Leopard Server), and it seems stable.