Called simply "Hatsune Miku" in its demo station, the game has you hit X and O in response to on-screen prompts (X and O icons flying into them), along with timed touchscreen swipes and directional presses, while Miku dances around on the screen. You're graded on the timing of each hit, and if you miss, it pops up as "WORST" which hurts pretty bad, guys.
The song I played was called "Catfood." Did I mention that I was surprised to see this in North America? There was no release info, or really anything. Just a demo unit sitting there.
Update: Sega reps told Joystiq that it's not actually planned for North America.