This is my biggest question, too. If I can upgrade from Snow Leopard to Mountain Lion for $29 or less, it might actually be worth it. The upgrade from Snow Leopard to Lion, though, just isn't worth $29.
I have two laptops running Snow Leopard. My mini runs Lion because it shipped with it. If I do end up buying Mountain Lion, can I buy it once and install it on all three machines? Apps from the OSX App store work that way because of using the same Apple ID. Does the OS work that way?