The A-list movie-star incursion on Broadway this fall just got a little B-listier with the addition of Haley Joel Osment to the cast of American Buffalo, David Mamet's 1976 play set for revival in November. And we can't wait: For sheer envelope-pushing, neither Daniel Radcliffe's full-frontal horseplay nor Katie Holmes's Dawson-ization of Arthur Miller is likely to compare to their fellow ex-child star's profane verbal tussles with castmates Cedric the Entertainer and John Leguizamo — a duo whose characters entangle Osment's young, broke schemer Bob in a bluer-than-blue cascade of "cunts," "fucks" and other Sixth Sense-era unutterables. And all it'll cost Osment, 20, is the low, low price of a semester behind at NYU: