Not sure about space or how that would be implemented. The whole idea of making the world a limited planet that is round is feasible but would be fundamentally different from the current way worlds are generated. This may be (if it ever happens) a new world type. If so, then yes, flying to the moon would be possible.
In the short term, it may be easier just to make the moon another "realm". Support for "realms" has been confirmed. I wish I could find that thread on here for the life of me. I need to keep better track of these threads that have good info from Red. But in it, he did mention at minimum API support to essentially teleport your character to a different world. This could be a second world on your server or hopefully even a different server. Some other server owners and I would like to set up a way to teleport between trusted servers. If Red doesnt add official in-game options for this, he said he would atleast add the functionality to the API so we could write our own plugins.
Just for clarification, realm hopping would be similar to what you could do in Minecraft with the normal nether portal or using some of the plugins available such as Twilight Forest or some of the others... "pocket dimmensions" or doors something. I can't remember the exact mod name. I know its bundled with Tekkit.