Now I don't want to seem ungrateful but I just don't see why for the time being it can't be a singleplayer. We've got the client, and I know it's unfinished but it'd be great as without connecting to a server and other players means less lag. Though I can see why devs wouldn't want players to be able to just blow through every quest. Eh, guess I'm just disapointed by how little I was on. Still it was fun while it lasted.
From what I've read above and elsewhere, the client doesn't contain any of the maps or NPCs. Without the servers up, the client has nothing to display.
For the client to work locally, the devteam would have to design and add to it a (probably very small) local map. That might more than double the size of the client, and be useful for little more than practicing with the controls. (Not that I think that would be a bad thing.) Even the player ponies have their data stored on the server, so no "leveling up" while running local; you'd probably get your choice of generic ponies in each of the three races.
And that assumes it's possible and the devteam thinks it's worth the effort while the game is still in Alpha development. If they even consider it, I wouldn't expect to see such a feature before Beta.