Wow, many great references, thanks to everyone who posted thus far. The literature on the topic is extensive, but in my work I find that all too often open source (a 'functional' good) and open content (a 'cultural' good) are lumped together. I tried to sketch out their differences w.r.t. motivations and consequences for organization and licensing in this article I published last year:
From open source to open content: Organization, licensing and decision processes in open cultural production Decision Support Systems, Volume 47, Issue 3, June 2009, Pages 229-244 Giorgos Cheliotis
Also relevant is the work by Bob Kraut and his team at CMU on motivations to contribute to online communities. Best, Giorgos Giorgos Cheliotis Assistant Professor Communications and New Media Faculty of Arts and Social Sciences National University of Singapore