This is from an article I gave to my daughter's preschool teachers and told them how grateful I am that they take this approach. It's a Montessori style preschool. As part of exploring without constraints they give DD4.5 books that are at her level. They've ruined me for other schools!
The full article is here:

http://www.cnn.com/2008/HEALTH/family/08/27/gifted.kids/index.html

Quote
Most educators believe that kids don't benefit from academically oriented preschools, either. Far more important is having opportunities to explore without constraint -- and teachers and parents who know how to keep learning fun.

"When it's fun and playful, that's when it gets into your head," said Robin Schader, Ph.D., parent resource advisor for the National Association for Gifted Children. Neuroscience research confirms that pleasure is what makes our brains want to repeat and remember an activity, and it's that kind of natural repetition that fuels learning.

This helps explain why play is everything to young children. It's how they learn, experiment, tinker, express creativity, work through feelings, practice socialization, develop language and math skills, and see the world in new ways. Pre-schools should mainly be play schools, centered on this kind of discovery learning and the teaching of basic social skills.