Example is the NYT apparently re-introducing paywalls in 2011, at least according to this NPR story: http://www.npr.org/templates/story/story.php?storyId=1227770...
Here is a blog post about the economics of the NYT paywall: http://blogs.reuters.com/felix-salmon/2010/01/20/the-economi...
I haven't heard about this for a while, and it looks like the posts I found are from the beginning of this year. Does anyone know if the NYT is still doing this? I hope not...