excerpts http://exploreit.net/improvethought/flow1.htm)

Flow: The Psychology of Optimal Experience,' (Harper & Row, 1990)

'Finding Flow: The Psychology of Engagement with Everyday Life' (Basic Books, 1997).

etc+

The activity must present just the right amount of challenge: If the activity is too hard, we become frustrated; if too easy, boredom ensues.

There must be clear and unambiguous goals in mind.

There needs to be clear-cut and immediate feedback about the activity's success.

Finally, our focus during the activity must center on the present ñ the activity itself.

http://www.apa.org/monitor/jul98/joy.html

http://hotwired.lycos.com/collections/web_development/4.09_csik_pr.html

http://www.unrealities.com/essays/flow.htm

Interfaces for Staying in the Flow » http://www.acm.org/ubiquity/views/v5i27_bederson.html