Hi!
"Cooperation can flourish if the public-spirited majority can punish freeloaders, say Swiss economists. People will pay to punish - suggesting that their notions of fairness outweigh selfish considerations. The work may help explain why people cooperate in society." tells a new Nature article:

http://www.nature.com/nsu/020107/020107-6.html
"Prosperity through punishment"

PS: Funny. Personally I do not like punishment, but giving gnutella members a feeling of "we do not tolerate freeloading" and "punish" them with friendly anti-freeloading countermeassures" (force swarming) and more control (chat/showing generosity) could also improve cooperative behaviour = better sharing. It's an emotional aspect, the article above could improve our understaning of human cooperative behaviour.

PPS: Also a Slashdot and infoAnarchy article now, see:
http://slashdot.org/science/02/01/10/0324223.shtml
http://www.infoanarchy.org/?op=displ.../11/02517/7790