A quick search on http://searchyc.com/pg+disagreement gets me the following pg comments:

http://news.ycombinator.com/item?id=117171 "I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness." - pg, 1150 days ago

http://news.ycombinator.com/item?id=392347 "Downvoting has always been used to express disagreement." - pg, 852 days ago

http://news.ycombinator.com/item?id=658691 "IIRC we first had this conversation about a month after launch. Downvotes have always been used to express disagreement. Or more precisely, a negative score has: users seem not to downvote something they disagree with if it already has a sufficiently negative score." - pg, 665 days ago