If you're not already running the developer preview but want to get your hands on OS X Mountain Lion as soon as you can, now is probably a good time to check whether you have a compatible Mac.
Mountain Lion has more advanced graphics requirements than Lion, so if you have an Intel Core 2 Duo MacBook from between late 2007 and late 2008, the mid-2007 Mac mini, a late 2006 polycarbonate iMac or the original early 2008 MacBook Air then you won't be able to upgrade.
As an aside, it's worth noting that Apple is also calling time on the 'Mac' prefix for OS X, confirming to The Verge that its preferred full name is "OS X Mountain Lion".
A quick scan of Apple's marketing and press materials show that Apple generally referred to "Mac OS X Lion" when talking about Lion specifically, but simply "OS X" when talking about the general platform.
Most materials in Apple's press site refer to "OS X Mountain Lion" but we have found one or two references to "Mac OS X Mountain Lion", specifically in the images section of the UK-focused press release about Mountain Lion, though you'll notice that it reads simply "OS X Mountain Lion" in the US release.