Fake gravity is kind of already added - if you make a large spinning station, and you make a large room near the rim you should experience something (not if the room is sealed, and not if you are already on the outer wall).
The problem with it is, whichever orientation the space station is spinning in, the fake gravity should be outwards if it's to be realistic. As I've posted in other forum posts recently, outwards gravity will be very unpleasant / hard to play for players, because the blocks must still be in their normal orientation. So the player will be pressed to the "walls" of the space station. We can turn the player around so he can stand upright on the walls (similar to what Star Mine does) but then all the regular blocks in the base will be on their sides. We don't think players will be too happy to have furnaces + other machines, bookshelves, enchantment tables, water tanks, crops etc all on their sides, it will just look "wrong". In testing we have also tried turning the blocks all around - so rotating the whole chunk - but then you get problems with connections (for example, wires and pipes and flowing water) at the chunk edges. There is no way to do this correctly in Minecraft due to the way chunks are. At least, the only way it could be done would be to have something like bedrock at some of the chunk boundaries, so that all wires and cables and redstone circuits etc, anything connecting block to block, would have to stay inside 1 chunk, and we don't think people will like that.
So instead we are currently looking at the possibility of having fake gravity whose strength depends on the spinning speed of the space station and the radius (how far out from the centre you are) but whose direction is still in the regular gravity down direction. That will mean all the vanilla Minecraft and modded blocks can still be built in a space station and will work normally.
This is, basically, why the space stations only spin horizontally. It will make no sense for the spin to be vertical if the gravity direction will be downwards.