I'm not a stick in the mud but thanks for teaching me that expression.

Here's how I came to the conclusion that use in own products is prohibited:

1. Look at This submission on Sketchup. The only indication of terms is the footer link to sketchup.google.com/3dwarehouse/preview_tos.html. Find a section in this document that permits the users of the Service to use the works outside of the Service. There appears to be none.

2. Look at http://sketchucation.com/forums/viewtopic.php?f=79&t=38857 and http://blenderartists.org/forum/archive/index.php/t-131594.html and copy parts of the text, for example "distribute copies of derivative works" - which seems essential to the interpreation of the users that use outside the services is permitted - and try to find that exact wording on sketchup.google.com/3dwarehouse/preview_tos.html and google.com/intl/en/sketchup/3dwh/tos.html . It seems impossible to find.

Works taken from or based on works from 3D Warehouse can not be released under free licenses without explicit permission by the copyright holders. This also means that they can't be uploaded to OpenGameArt.

Good luck with your project but I advise you to talk to people you trust to interpret a legal document for advice (and not rely on years-old forum posts by strangers).

The terms prohibit re-distribution and modifications outside of 3D Warehouse.

The terms permit re-distribution and modifications only inside of 3D Warehouse.

(1) The "preferred form for making modifications" part of the GPL (with respect to art) is at best vague and at worst completely unenforceable.

"preferred form for making modifications" is also vague for code. Vague is good, because it can include technologies and practices that do not yet exist. Somehow it appears to be clear however, that "preferred form for making modifications" for example means non-obfuscated code. Whether this also means that comments which help understand derivative GPL code may not be stripped and whether having to provide a revision control history of derivative GPL code - when it exists - can be challenged by the owners of the original GPL code and ultimately clarified in court.

For example, let's say someone makes a midi file and uploads it to the site.  Someone else (who may not have access to mixing software or even a computer at the time of recording) takes the midi file and records themselves playing an accompaniment to it.

To compare to a code situation: what if you take a compiled GPL program and start making binary modifications to it using a hex editor? The case becomes uncommon and the interpretation of "preferred form for making modifications" becomes unclear to many (including me). I would assume that the source code of the binary and documentation on how the binary was changed might be enough.

Of course modifying 'object code' is much more commen in digital art, which often makes the requirements non-obvious.

So now you have an audio file that's a live instrument with the midi playing in the background.  What's the preferred form for making modifications?  Can the GPL be construed to prevent this sort of modification?

The preferred form for making modifications would have to be determined. This would most likely make creators of derivative works ask the GPL original owners and/or other creators of such works for clarification what would be considered the preferred form. By that I mean not that they should ask "I want to edit this: how do I have to edit it and what do I need to provide as the preferred form for making modifications" but instead "I edited this art in such and such way. What is considered to be the preferred form for making modifications?"

If the GPL [...] doesn't prevent modifications like this, then the clause about preserving a perferred form for modifications just gives the original rights holder a false sense of security.

The clause does not mean "you may only use methods which leave access to the most preferrable form of making modifications". It means "when providing derivative works, you need to provide the preferrable form of making modifications". What that is, depends on the methods.

In either case, this sort of ambiguity makes the GPL a non-optimal choice as an art license.

The ambiguity allows for works to be used and edited in ways that have not been considered or have not been konwn to the authors of the GPL and to the owners of the GPLed original work.

That being said...

I consider the GPL to be non-marketable to many artists because there is no preamble that speaks to creators of works other than software (because the word "software" is used all over the preamble) and because there is no "How to Apply These Terms to Your New Digital Art" information (there is one for code at the end of the license page).

On the other hand there is no other sophisticated strong copyleft (strong meaning that the preferred form for making modifications has to be provided) alternative, which makes GPL clearly useful to those artists who want this requirement.

To confirmation of the statement "The GPL and LGPL aren't appropriate licenses for art." kind of devalues the licensing choise of the users that pick it.

If OGA's position is that GPL is not a license in the interest of artists, then there should be a warning or the option to chose it even disabled.

"(now covered by CC-BY-SA 3.0)" sounds like in version 2.5 and earlier there was not SA option (I guess just removing the version number will solve that) 

(2) That isn't a mistake, just poor phrasing on my part.  What I was getting at is that if someone wants to license their work under the MIT license, which isn't an available selection, just select the GPL and then note in the comments that they're also releasing the work under the MIT license.

Please change "GPL" to "MIT". I understand this will make the phrasing clear?

Thank you very much for replying so extensively!

I have no art education. To me the line between intended concept art and potential story backdrop or potential character portrait varies on the work.

I like this mockup (I hope this term works as a replacement for "mock in game screen shot"?) and OGA needs more mockups. It might get complicated to ensure license and attribution requirements are met when different sources are mixed. But still: yay mockups!

To me, the concept of mockups seem more related to game design documents than to concept art. On the other hand, the documents section here http://opengameart.org/art-search-advanced?keys=&field_art_type_tid%5B%5D=11 is chaotic.

I look forward to the individual assets!