Here's a thought to throw out there for affiliate marketers. I think we can probably assume that Google knows how to identify an affiliate or agent site in many instances. It probably knows how to recognise a brand and it probably uses the knowledge of each of those to detect the other.
There are of course legitimate reasons for an affiliate site to link to another but when you think about your own site, how many links have you put in place to another affiliate site purely because it's a link that will interest your users and not one that is influenced by commercial reasons?
I wouldn't mind betting that this is something that has occurred to Google.