The issue of compatibility between the Creative Commons licenses and that of CGTextures.com came up in another thread. Since copyright and licensing is a complex and confusing issue, I decided to fork that conversation over here so as not to take the thread terribly off-topic and to make it easier for people to find information about licensing.
Redrobes pointed out that the CGTextures license specifically forbids releasing derivative works (new artwork incorporating their photography) under any kind of open source license. The immediate ramification is that at least one of the two maps I submitted to the CWBP may not be lawfully permitted in that project. I used at least two CGTextures images in my Mennin's Hallow map, so it is not eligible to be released under an open source license. Since the entire CWBP is licensed CC Share Alike Non Commercial, Mennin's Hallow cannot legally be distributed if CC is considered an open source license.
As a side note, that same CC license means that GamerPrinter's Kaidan may also be in not-quite-legal territory, since it is derived from the CWBP project. Likewise, it is unlawful for any of us to sell prints of any map we create for the CWBP. Not that anyone around here is likely to sue over it, but I thought I'd point it out. I've already issued an additional license for my two maps and accompanying text to permit commercial use of derivative works; I don't know if anyone else has done the same.
Now, on to my real confusion: Is a CC license an open source license? As I understand it, it is not. CC makes available the finished artwork, but not necessarily any of the source elements that went into creating it. For instance, a CC license on a piece of music created with Sony Acid does not (as far as I am aware) make available to the licensee any of the loops used in the creation of that piece, except where they can be extracted from the finished music. In contrast, GNU is truly an open source license—every element used in the creation of a GNU work must also be able to be licensed under GNU.
Now, as I said in that other thread, I haven't read any of the legalese in the CC licenses as of yet, so if there are relevant clauses that we should discuss, please post an excerpt or a link to the relevant section when making a reply. I'll try to get in there and look at it myself, too, so that I can have a more intelligent conversation about it.