Two days ago (November 1, 2013), I upgraded my Mac Mini from 10.6 to 10.8. I bought Mountain Lion two days before that as a download ($19.99) from the Apple store at http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion . It was a bit tedious to do so—I had to enter my Apple ID and password six times on six different pages before I could check out.
The next day I got a redemption code that I could use with the App Store application to start the download. It took just over seven hours, but I live in a rural area where broadband is not available.
The installer expected 10.6.8 to be on the disk; mine was an earlier version. So I had to apply all the Snow Leopard updates. Then the Mountain Lion installation succeeded.