Please note that this has not been confirmed by Summit Entertainment. 

The blogger, TwiCrack Addict has posted a memo from Production Weekly dated 02-12-09 that has Dakota Fanning as part of the cast list.
 

“THE TWILIGHT SAGA’S NEW MOON” Feature Film 02-12-09

VOLTERRA FM PRODUCTIONS CANADA INC.

STATUS – March 23 LOCATION – Vancouver – Italy

PRODUCER: Patrick Wachsberger – Wyck Godfrey WRITER: Melissa Rosenberg DIRECTOR: Chris Weitz LP: Bill Bannerman PM: Barbara Kelly

CAST: Robert Pattinson – Kristen Stewart – Taylor Lautner – Billy Burke – Gil Birmingham – Peter Facinelli -Elizabeth Reaser -Ashley Greene – Jackson Rathbone – Kellan Lutz – Nikki Reed – Dakota Fanning

You can view the post here. Read the original post on Twilighters.org