Yesterday Mojang released the Minecraft Pocket Edition for Android OS. It’s currently only available on the Xperia Play for a limited time and will be released to other Android devices after that. It costs $6.99 and does support multiplayer over wifi. It’s more or less “Creative Mode” on a portable device as it doesn’t look like there’s any combat or monsters in this version.
Jeb, one of the Minecraft developers over at Mojang was asked if it was coming to iOS devices and he replied “soon” (source).
I’d personally like it if it was created large enough for the iPad as well as ported to the 3DS. I’d also like to be able to move the worlds from my PC to my gaming device. That way I could grab a single player world, play it a bit while I was out, and put it back on my PC and pick up when I get home.
It’s available on the Android Market.