This is the key difference. Stack Overflow has said all along that it would always be free and the CC license helps ensure this.

If Stack Overflow decides to put up a paywall anyone could simply put up the all existing content, perhaps using one of the existing open source SO clones [1]. Also, a lot of the current Google traffic would instead go to one of the sites which currently legitimately repost all of SO's content [2].

[1] http://meta.stackoverflow.com/questions/2267/stack-overflow-... [2] http://meta.stackoverflow.com/questions/24611/is-it-legal-to...