StingingVelvet: They said Windows 8 as well, though maybe that is just for social features?
No idea, honestly, but everything cross-platform and developer-related will be revealed during Build which will happen in a few weeks. Build is their developer-related event that lasts a few days. We'll know then for sure.
This seems like quite an interesting concept, especially for kids, if they can easily share worlds and play others' worlds... something like Minecraft, but prettier and on bigger scale. Also I noticed that AI can be developed to govern the actions of NPCs in it... hopefully this is true, would be very interesting if it is TES-like complex when you get down in details.