Carrie Fisher is the first celebrity guest confirmed. "Princess Leia is going to be there, and she's always a huge, huge hit," says Steve Sansweet, Lucasfilm's head of fan relations.
Interestingly, the article refers fans to the CV website for more information on the announcement, but so far the website has not been updated with details. Additionally, given that Official Pix's Ben Stevens said today that the first guest would be announced tomorrow, it's possible that the USA Today story was published prematurely.
The article, found here, also discusses new toys commemorating the 30th anniversary of The Empire Strikes Back, including the mail-away Boba Fett and the AT-AT.
UPDATE: A story with this announcement hit the official site shortly after I wrote this post.