Game designer and author of books on the subject, Chris Bateman just posted on his Only a Game a very good article about Paidia, term coined by Roger Caillois to designate the more anarchic, free-form aspect of play permeated by, as Caillois writes, 'diversion, turbulence, free improvisation, and carefree gaiety'. (On the other side of the spectrum is ludus, much more based on rules and requiring more skill and effort from the player. Needless to say, nongames are much more related to paidia than to ludus).
In his article, Bateman says:
´I have accused game designers of being remiss in overlooking the value of alea (games of chance) but we are, on the whole, prone to overlook paidia completely. This is not surprising: the game designer's craft is generally about producing the framework of play, which is to say the rules and abstractions that define the game world and its gameplay. In essence, the game designer works in the field of ludus, and this application of ludic elements is a contrary state of affairs to paidia.´
That could help to explain why one of the most sucessful nongames ever, Electroplankton, was created by a multimedia artist and not a game designer. However, I do think paidia is getting much more space within the game industry than it used to. Just take a look at Nintendogs, Animal Crossing and, to avoid a Nintendo monopoly on this post, The Sims.
Here is the link to the article.