I wonder what license the image was released under. While this could have been deliberate by Disney, I doubt they would use a (seemingly) very popular design on production material.
Many times that I knew some sort of stuff like this happened, it was because one specific worker, KNEW that he was doing wrong, but did it wrong anyway to take the credit, and hope his bosses would not find out.
This also apply when the "worker" is a boss, but working for a client, he do it in a lazy manner hoping his client won't notice...
Whoa, I'm really shocked that the case essentially lost there several times. That's.... terrible. One of them because they "failed to register copyright in the US" according to the Wikipedia. I was under the impression that there was no real need to register under US copyright law as its an automatic right.
I was under the impression that this would be covered under copyright and not a particular licence. As soon as the artwork was created by the artist it belongs to the artist, they don't even need to put a copyright symbol next to it. My advice would be get in touch with Disney, it seems the artist has support on tumblr already. I suspect a public spectacle of Disney's mistake (assuming it was a mistake) is probably the artist's best bet. DKNY were caught up in something similar and ended up paying and apologising for their mistake [1].
Copyright is the law from which licenses get their power. It's because the author has the exclusive rights granted by copyright protection that they can dictate the terms under which others may use their work (that is, offer licenses). When no license is offered, the author retains exclusive rights. The parent comment was implying that the author may have offered some kind of license (for example, by uploading his artwork to Flickr and ticking off one of the Creative Commons boxes in the license selection).
It's also quite possible that someone unauthorized put the picture on a site with a permissive license. That's an easy explanation for how this came about. I really do doubt that Disney or anyone representing them intended to steal this image.
Then again, things like this have happened.