Splatoon will have single player.

[MDMAlliance]

Nintentacle said:

It also needs 2v2 local multiplayer! 4 Wii Remotes/Pro Controllers/Classic Controllers, with someone using the GamePad for something else!

I'd also love it if you get to chat with your team on the online for a couple of minutes to make a strategy!

That would actually be hard to do given how the game controls.  While optional, the ideal control method is a hybrid between gyro controls and the analog sticks.  You also use the touch screen for some stuff as well.

edit: 2 Gamepad controls or finding some kind of work-around is possible.  I'm just saying it obviously wont be the easiest thing to implement.