I think it is more practical to think about ways to prevent conflicts than trying to separate right from wrong afterward.
Flickr should allow users on opt-in basis to display a clear unignorable warning sign to viewers that copyrights will be contested if photos are used without explicit permission because, as it is now, it's too difficult to distinguish wolves from sheeps.
Update: I think it also makes sense to do this at per-photo and per-photoset level. This would allow photographer to have a copyright-loose photos as well as 'pro' photos for sale.
What we have here is, IMHO, a communication problem, not a legal problem.