Currently, Stack Exchange sites use the CC BY-SA 2.5 license.
There's a lot of content (not the least of which is Wikipedia) under the newer 3.0 version of this license.
Content licensed under 2.5 is forward-compatible and can be used with the 3.0 license, but the reverse isn't true -- CC BY-SA 3.0 content can't be used here.
Is there a particular reason SE doesn't use the newer version of the license? Should it be changed?
As of around April 8, we re-licensed under 3.0:
There are some noteworthy differences between the two. 3.0 has a no endorsements clause, and it introduces the concept of a "Creative Commons Compatible License", a third-party license approved by CC as "essentially equivalent" (none are approved yet). The generic license was renamed to be the US license, and they made a new "unported" license (linked above) to fulfill a similar role. 3.0 is also approved as free by Debian. CC has a article and blog entry with more info.
Legal page says user content is under cc-by-sa-3.0, but other pages say cc-by-sa-2.5 (fixed)
