Nintendo of America's Bill Trinen demonstrated the game for press in attendance, showing unique fielding mechanics in which shaking the Wiimote signals the fielders to go, and a throwing motion sends the ball to the correct base. Pitching and batting seem to be pretty much what you'd expect!
According to IGN, the cast will include the usual suspects: Mario, Luigi, Peach, Daisy, the Wario Bros, Donkey Kong, Baby Mario, Toad, Toadette, Boo, and a Shyguy. More details about Sluggers should come out sometime next week.
[Via Joystiq; press release available here]