The idea of an "planet simulation" (i.e. when I walk for a given amount of time into one direction, I will finally end up again at the spot where my journey started) is quite interesting, but there is one problem: It's difficult to find a good size for the world. It is more complicated for us to implement such a system than simply implementing a "real" endless world, but it would be a pity if the world is still too big that nearly no player would realize that it is spherical.
Otherwise, if the world has a fixed size, there could be indeed not enough room for big multiplayer servers, and apart from that, all our effort we already put into preparations for an "unlimited" world would be a waste of time ![]()
So I think a (horizontally) unlimited world is the best approach (low priority though), but there will be a few exceptions: On the one hand, the stranded mode will most likely have a fixed worldsize, surrounded by an ocean. On the other hand, people will be able to restrict the size of their world in creative mode (this is especially interesting for creating adventure maps).
(Currently you can btw. already restrict the world by using Lua scripts)