Recently I have been evaluating licenses for user generated content for a new project. I was disappointed to see that most sites with user generated content have arcane, legalistic terms and conditions and that there is no consistency from site to site. The problem when you don't use a well known license is that you end up having to make convoluted explanations and your users are still confused no matter what. Check out paragraph 9 of Yahoo's Terms of Service to learn what rights you have to any photos you have uploaded to Flickr or this news.com article where Google clarifies its terms and conditions for your Google Docs documents.
Since the Creative Commons is a repository for authors and artists to share their work, I checked out their license to see if it was relevant. The creative commons license is cafeteria-style and is perfectly suited for user generated content! I have long been a big fan of commercial friendly licenses and have released a lot of my early code into the public domain, with the Java license when I was at j.rad, and with the Apache license when I was at ActiveGrid. Therefore, my inclination is to go with the Creative Commons Attribution (by) license, which allows commercial use of works with attribution. In open source terms, it is most like MPL+Attribution.
I am pleased that the open source principles that software engineers know have made it into the content world, and that we will be able to use a credible, understandable, and well known license for content in our new project!