Tyler Posey MAY be Seth, blogged Koa Orquia(New Moon hopeful) on his MySpace. 

Tyler Posey as Seth! :D A New Moon Hopeful, Koa Orquia, has blogged on his myspace that Tyler Posey MAY have been cast as Seth Clearwater, here is what he has to say-
parole chiavi: New moon, seth, jacob, Lupi, quil, embry, sam, jared, paul