There has been much debate and speculation over the past weeks, as to who will be playing Leah Clearwater. Summit are still keeping quiet as far as the casting of Leah and Seth goes, so it's been left up to the media to wonder... I was just surfing YouTube and found this partial podcast interview by Twilight Source about her audition experience and why she loves the character so much. For anyone who doesn't know much about Q'Orianka Kilcher, her IMDB page can be found here.
There have also been rumours circulating, that Julia Jones (imdb page here) has been cast as Leah, but there has been no official confirmation on either actress.