I'm thrilled to see Rittel's work acknowledged (he's my old professor); but just to pick a nit, the paper which is generally considered to have introduced 'wicked problems' was a paper he wrote with another professor of mine, Mel Webber - "Dilemmas In A General Theory Of Planning". It's available <a href="http://www.uctc.net/mwebber/Rittel+Webber+Dilemmas+General_Theory_of_Planning.pdf">online as well.