This is a kind of stroke of ironic luck on Fox's part as head of Fox 2000 (the part of Fox that will be spearheading production and development matters) had been pursuing the Narnia franchise but was beaten to it by Walden. Over the past few years they have been looking for "a family-friendly, lit-based franchise for years," with Eragon being a disappointing financial flop for them.
The sequel, subtitled Voyage of the Dawn Treader, is reported to have a $141 million budget, which is certainly a lot less than the $215 million that Prince Caspian had. Maybe they're playing it as safe as they can budget-wise (although no one's saying $141 million is in any way a small amount of money) as the high budget of the last one was the reason it was such a financial disappointment.
What may have contributed also to the box-office disappointment of Caspian was that it's considered to be the least commercially appealing of any of the original books and I'm sure the more family-friendly style of Dawn Treader will contribute to it doing better this time around.
The not too distant summer start date for shooting depends on the writer. The most recent draft that both Walden and Fox approved was written by Richard LaGravanese (The Fisher King, P.S. I Love You) but his availability is in question as he's currently adapting the book Water for Elephants for none other than Fox 2000.
I'm pretty sure Narnia is a bigger priority for the company, however...
After the wheels are in motion the film will likely be shot in Australia, moving away from the originally intended location of Mexico in the Fox owned facility Rosarito where such films as Titanic and Master and Commander were shot.
Okay, okay, I'll go ahead and say it - Fox probably has (or will have) enough money lying around after the WB/Watchmen settlement is done with and probably saw dollar signs when Walt Disney pulled out of producing this series. Although it's certainly a risk on Fox's part because of the financial failure of the last film they probably think the payoff if it does make as much money as they're hoping, is worth the risk.
So are you glad that the Narnia franchise has been saved and will continue on? And are you glad that Fox was the one to save it?