If you're thinking of starting a nature-based preschool you might be interested in an article I recently wrote for the Natural Start Alliance's March Feature Story. My article "Program Considerations for Establishing a Nature Preschool" can be found here. You might also be interested in another article related to starting a preschool program which was written by my colleague and friend Dave Catlin. You can find his article titled "Getting Down to Business" here.
I hope both articles are useful for you--the world needs more nature-based preschools!