Según movieweb:

After a little bit of confusion, it seems that the next Narnia movie will be lensing this summer. According to NarniaWeb.com, the site has confirmed with Disney that the third film, The Chronicles of Narnia: The Voyage of the Dawn Treader will go before cameras this summer for a May 7, 2010 release.

In an story from Variety, producer Mark Johnson told the trade that filming will start this November, for a May 2009 release. NarniaWeb.com says those dates are inaccurate.

Lucy and Edmund Pevensie return to Narnia with their cousin Eustace where they meet up with Prince Caspian for a trip across the sea aboard the royal ship The Dawn Treader. Along the way they encounter dragons, dwarves, merfolk, and a band of lost warriors before reaching the edge of the world.
Todavía no han estrenado la 2ª y ya planean la 3ª. Aún y así lo encuentro lógico. Son bastantes libros en la saga y hay que amortizarlos.