Obviously there is a great deal of interest in Zendikar and what Planeswalkers will appear in it. So far we know there are going to be three:

Chandra - Originally appearing in Lorwyn, Chandra's story has slowly unfolded and today on Magic the Gathering's Facebook page, they revealed the blank template of Chandra's new Planeswalker card.

Nissa - Nissa first appeared on the Xbox 'Duel of the Planeswalkers' and it is known that she will be in the upcoming Zendikar set. Her art will match the art we've seen on the cover of the Xbox game. She is from Zendikar and looks rather elfish based on her art.

Sorin - Sorin is a very old planeswalker, he's a vampire, and he's not from Zendikar. Those are the details we got from Mark Purvis when discussing him. He's obviously going to be a powerful and interesting character in Magic's new stories.