I really wouldn't want to play Minecraft on such a small screen. I can only imagine how it would make high areas feel when you endlessly climb up them. Plus I just can't see charging people for the portable version if they already bought the PC version. I never liked the idea of re-buying the same game on another platform.
I would not play minecraft at all. Why limit what I can do with my imagination in a 3d world? But that's another debate.
I think a MMO of any sort should not be played on any portable with tiny keyboard or no keyboard. Even when bored while on the bus. Let alone taking time away from the big computer with HD screens to play on tiny screens. But as long as there are people wanting it, there will be someone supplying it.
Thumbs up for the free market!