Under the creative commons license under which xkcd is published, while replicating the comics elsewhere is perfectly acceptable, not providing information of where the comic originated is not. Quoting the page itself...
This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License.
This means that you are free to copy and reuse any of my drawings (noncommercially) as long as you tell people where they're from.
That is, you don't need my permission to post these pictures on your website (and hotlinking with <img> is fine), just include a link back to this page. Or you can make Livejournal icons from them, but -- if possible -- put xkcd.com in the comment field. You're also welcome to use my drawings in not-for-profit publications (with attribution). If you want to use them in a commercial publication, or you're not sure whether your use is noncommercial, feel free to email me and ask.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License.
This means that you are free to copy and reuse any of my drawings (noncommercially) as long as you tell people where they're from.
That is, you don't need my permission to post these pictures on your website (and hotlinking with <img> is fine), just include a link back to this page. Or you can make Livejournal icons from them, but -- if possible -- put xkcd.com in the comment field. You're also welcome to use my drawings in not-for-profit publications (with attribution). If you want to use them in a commercial publication, or you're not sure whether your use is noncommercial, feel free to email me and ask.
from http://www.xkcd.com/license.html