Will a Mac with OS X Mountain Lion be able to upgraded directly to OS X Yosemite when it's released, or will it have to be upgraded to Mavericks first, and then to Yosemite?
I'm thinking you would need to go to Mavericks then Yosemite. I personally start fresh when a new OS is released to ensure I'm not carrying over any bugs, bad settings, etc. So that may be the route you'd want to take.
if a Snow leopard machine capable of running Lion and wanted to "run" Lion then it would need to update 10.6.8 to gain MAS .. ML to Yosemite upgrade process will work if your Mac is capable of running Yosemite ...