There is now an Open Clip Art project:
The only license they use is Public Domain, which I understand from a commercial point of view, but it would have been nicer to have a choice of the Creative Commons licenses. I’d add my work if I could CC it.
There’s nothing to stop you taking the work and combining it with your own work to make something that you then CC. That’s the disadvantage of the Public Domain: you can steal from it. But this can work for value creation as well as for asset stripping.
I really do think there’s a need for a creative equivalent of SourceForge; a host for creative projects. Computer game projects put graphics, sound and other content under version control, so artists can certainly do it. For Open creative projects, keeping track of submissions and being able to roll them back is vital to avoid any copyright infringement problems.
If only I was any good at writing NESTA funding proposals. :-)