The main CC licenses have been released in four versions: 1.0, 2.0, 2.5 and 3.0. References to changes associated with each version will follow.
Initial version, released 2002-12-16. http://creativecommons.org/weblog/entry/3484
Released 2004-05-25. Changes explained at http://creativecommons.org/weblog/entry/4216
Version 3.01 was discussed 2007-10, work will be folded into a future version, possibly a version 3.5, proposed 2007-12, possibly a version 4.0 over a longer time period (3.01 and 3.5 are not now in active discussion and there may never be CC licenses with those version numbers).