A recent article in Entertainment Weekly has outlined Walden Media's plans for filming the rest of the movies. The order is as follows:
The Lion, The Witch, and The Wardrobe (2005)
Prince Caspian (Christmas 2007, maybe Christmas 2008)
Voyage Of The Dawn Treader
The Silver Chair
The Magician's Nephew
A Horse And His Boy
The Last Battle
The article states the Prince Caspian may be pushed back to Chistmas 2008 due to visual effects. Equally, Caspian, Voyage Of The Dawn Treader, and The Silver Chair will be filmed back to back similar to the filming of all three Lord Of The Rings films. This does make sense as the stories take place about a year between books (our time not Narnian time), and I'm sure they want to keep the kids at a youthful age and not twenty somethings portraying preteens.
As for the last three books why the order selected remains a mystery. I'm going to assume that it will give the actors who portray the four Pevensie children to catch up in age to their counterparts in A Horse And His Boy and The Last Battle. A Horse And His Boy takes place during the time frame between the coronation and the hunt for the white stag at the end of the Lion, The Witch and The Wardrobe. While The Last Battle takes place approximatly ten years after Pevensies first enter the wardrobe. If this is the plan for filming then I'm happy with it as we don't want another Harry Potter situation where the kids look nothing like their age.