From IGN.com:
Prince Caspian Seems Certain
The Pevensies return to Narnia.
by Paul Davidson
January 6, 2006 - The Chronicles of Narnia: The Lion, the Witch, and the Wardrobe is enjoying a fantastic theatrical run and proving to be even more popular than its competition – notably Peter Jackson's King Kong remake. If there was any question about a sequel before, it's pretty much been answered.
Word is that Prince Caspian, the next logical story in the series, has been given approval. The story takes the four Pevensie children back to Narnia, summoned by Queen Susan's horn centuries after Narnia was freed from the White Witch's spell.
NarniaWeb claims to have inside information that Douglas Gresham, co-producer of LWW and stepson to the late C.S. Lewis, is already at work on the next film. Another website, Dark Horizons, quotes an official Walden Media message board post as confirming that the four young actors and actresses have all signed onto Prince Caspian, and Andrew Adamson has agreed to direct again. Adamson has strongly hinted at returning for the next sequel or two, but given no firm commitment to Prince Caspian in public.
The clock is running, due to the necessity of making the next few films while the children are still young. Expect to see more official announcements in the coming weeks, and an expedited production schedule.