We are sitting here watching Prince Caspian, and I looked up the Wikipedia page for the next movie. It looks to me like they have choosen a date for its release:
Wikipedia: The Chronicles of Narnia: The Voyage of the Dawn Treader
From what it says, we have to wait a year to be able to see it:
Filming is now complete, post-production has begun and the film will be released on December 10, 2010 in the United States and United Kingdom.