To inaugurate this blog about educational games, here's a paper I just happened across that grapples with the concept of fun in the context of software design.

http://www.psy.gla.ac.uk/~steve/fun.html

The premise of the article is that there are three different cases in which it might be important to concern yourself with making software "fun": 1) where enjoyment is the main function of the software; 2) where learning is the main function; and 3) where learnability is considered an important secondary requirement of software with some other main function. Seems to make tentative sense to me.