The 2008 release is fine as long as they get the actual filming done before all the actors are incredibly old for their roles. They were very smart in choosing to do the next three films back to back so they won't meet the problem with cast ages with Skandar and Georgie.
William is already 19, and they will be filming probably at the end of this year into next year (if Pippa Hall hasn't begun casting for Prince Caspian and the screenplay isn't done, it will be awhile before they start). He is playing someone who, in the book, is 14. But after Prince Caspian, he will no longer be in the movies, nor will Anna, so we don't have to worry about their ages.
I'm just hoping they use the extra time to make it an even better film. As long as I'm satisfied with it when it does come out, I'm ready to wait a few months longer than planned.