The Content You Use Today May be Licensed Differently Tomorrow.

In Verifying the Commons, the Creative Commons folks point out the tricky subject of attribution licenses converting to traditional copyright. What happens if you used a photo under a CC attribution license and the photo was subsequently re-licensed as fee-for-use? How do you prove that you didn't pirate it? The post points out that there are multiple ways to timestamp or otherwise verify that you first utilized the photo during the license period (note that CC licenses cannot be revoked, so anyone who used content in the "open license" window retains the right to use the content):

As for the question of verifying whether a work was ever released under a CC license, the innovative ImageStamper.com can provide this exact service for flickr photos. We used ImageStamper to time stamp all 157 photos used in Jesse Dylan’s ‘A Shared Culture‘ so that we would have proof, going forward, that a particular work was released under a given license. WebCitation.org’s archive feature provides essentially the same functionality for any given webpage and also provides a permanent URL for the snapshot.

Useful to remember that we're not only responsible for attribution, but for verification of the license. Even in the goody goody world of shared content, you need to watch your back.