We've been running the co-op on one of our sites for about a month and saw some steady increases in rankings until this last week. Some pages we've ranked #1 in and top 5 for months now have dropped as far down as 100+ in the last week or two. In the link that WRMINEO listed previously thy specifically state that they don't like "Using techniques to artificially increase the number of links to your page, such as link farms." I wonder if they are starting to penalize co-op?