I think a lot of designers would love to work on open source projects, but I'm a designer and a developer myself and even I feel that the creative input wouldn't be taken seriously.

If an OS project assigned a Creative Director, that may help lead the way for other designer to join a project.

Also, Source Forge and GutHub and the like aren't going to cut the mustard for Designers. There needs to be somewhere like them, but tailored for design.