To answer my own question. The latest issue of MacWorld has an article re: Mountain Lion. This is from that article:
Will I need to be running Lion to upgrade to Mountain Lion, or can I upgrade straight from Snow Leopard?
According to the information Apple has provided with the developer preview, you need to be running at least the latest version of Snow Leopard (OS X 10.6.8) to install Mountain Lion.
For more info:
Mountain Lion: What you need to know