Towards a theory of fun

I'm reading Edward Castronova's book Exodus to the virtual world, which will be on the recommended-reading list for my workshop this summer. Castronova is an economist, but is actually interested in people rather than just money.

The book's subject is how RL will have to change in response to the time and money that we virtual-worldists are not spending there. Castronova spends quite a of time defining play and fun, and how these are encouraged in virtual worlds.