License for your authors. Also on performancing.com

Some of are thinking about starting a blog network. Some just allow everybody post on their site (like Performancing.com does).
Have you ever thought what right do these authors transfer to you and what they don't? What if you are making money from their free contributions? What if you are not doing it now, but in five years you will publish "The early days of blogging" offline and use their posts?

I carefully examined the Performancing copyright page. Guys are very precise about what you can and cannot do with their content, but don't say a word about content by third parties posting at Performancing.

I am also not seeing any copyright-related message while I am typing this post. It means that I am retaining all the rights to get money from it, doesn't it? How comes that after I click "Submit" this post as any other site content is "licensed under the Creative Commons Attribution NonCommerical License"?

Please, don't get me wrong. I love the Performancing and if asked for will allow to use this post for commercial purposes :) I am worried about my potential blog network and I am interested in knowing how others solve similar issues. After all, it should be rather common issue, shouldn't it?