The footer on xkcd.com says "This work is licensed under a [Creative Commons Attribution-NonCommercial 2.5 License](http://creativecommons.org/licenses/by-nc/2.5/)", so I asked him on IRC if it was fine to use the 3.0 license and he said 'sure'. I'm certainly not beholden to any license, and I leave it up to him to change.