"...I actually haven't seen very much discussion on this."

...Really? I think there's been a lot of discussion on this. Nothing was ever truly concluded on the topic, though. Here are some stuff on GPL as it relates to art: 

http://opengameart.org/content/faq#q-ccgplcompat 

"...the Free Software Foundation has clarified that the game code and game media are separate entities and do not need to be released under the same license, provided those licenses allow you to copy and redistribute the work for both commercial and non-commercial purposes..."

http://www.gnu.org/philosophy/free-system-distribution-guidelines.html

"Non-functional Data

Data that has an aesthetic purpose, rather than a functional one, may be included in a free system distribution as long as its license gives you permission to copy and redistribute, both for commercial and non-commercial purposes. For example, there are some game engines that have been released under the GNU GPL, and have accompanying game information "world map, game graphics, and so on" released under such a verbatim distribution license. This kind of data can be part of a free system distribution."

http://opengameart.org/forumtopic/source-required-for-art-licensed-under...

"...The reason the GPL is a license option at all is because there were several major projects that, at the time of OGA's creation, required their art to be licensed under the GPL (Wesnoth comes to mind, although I believe there were others), and because of the existence of legacy works from back when the GPL was the only game in town as far as licenses with a "share-alike" requirement..."

 

There is a bunch of threads in the forum talking about GPL as it relates to art, but they are usually more like blurbs mixed into a larger (and less related) licensing discussion, so it's a bit hard to wade through it all. Capbros has written a lot on the topic and he definitely understands GPL better than I do. Maybe he'll see this and give his assessment. 

TL;DR: My overly simplified summary of the issue is:

• GPL is not the recommended license for art, but there are legacy reasons for maintaining it as an art license.
• GPL'd art doesn't neccessarily mean the code must be GPL as well. Art and code in a single project may be licensed separately as independent "collections". However, each project is subject to unique circumstances. Consult your lawyer if swelling persists longer than 4 hours.
• If you're submitting GPL artwork, it is preferable (and polite) that a highly-editable format of the art be included along with a more finialized form. E.g. include a photoshop PSD file (or gimp XCF file) along with a PNG or JPG image of artwork, so other users may modify the image easily. Whether or not this is required in order to be compliant with the GPL is unlikely/debatable, but still a good idea.

Hope that helps some :)

'. For coding, I believe a GPL license means that all new code must be licensed under GPL or another compatible license (i.e. something more permissive such as BSD or even public domain), 

absolutely not.  Derivatives of GPL code must be released as GPL period.   There's no provision for releasing derivatives under 'compatible' licenses.  Otherwise you could trivially flip something from GPL to CC0 and then to a private/closed source license.

As for art, MedicineStorm pretty much summed it up as far as my research has taken me.   To be honest, I can say I've studied what GPL means for artwork some but I wouldn't really say I understand it much.   My general take is that the license wasn't written for artwork and therefore shouldn't be used for it (this is the reason for the CC folks getting started).  But people do release art under GPL and the FSF does seem to have an idea that the license can work for art, so clearly there's more for me to learn on the topic.

CC0 would be sufficient... wait. The artwork being submitted is from a GPL project? If the artwork is already GPL and you don't have authority to change the license (you're not the artist) then it should stay GPL.

Ah. Yes. As the author of such artwork, you are free to release the artwork under any number of licenses you want. GPL over there, CC0 over here.

CC0 would be sufficient to cover all future bases. People coming here can use your CC0 art in a GPL project, a CC-BY-SA project, or a closed source commercial project. :)