Rather, I suspect that the reason more graphic designers don't contribute their own talents to open source projects is much more mundane. Simply put, graphic designers aren't aware of the shortage of talent because graphic designers don't use open source software.
Have you seen the GIMP? While it's often billed as the "open source Photoshop," it lacks many of the tools that professional graphic designers need for print production. Even worse, its UI is about as handsome and user-friendly as a piranha. What self-respecting graphic artist could tolerate it?
The maintainers of OpenOffice.org have come up with one potential solution to the open source graphic drought. They're sponsoring a contest to come up with the best free document templates and clip art to be bundled with the open source office suite. But this is only a partial solution at best.
Unless the community can attract a broader audience to open source software -- including graphic professionals and other nondevelopers -- we'll never escape the catch-22 of open source UIs. Until open source applications can appeal to an audience with sophisticated aesthetic tastes, they will always look and feel like exactly what they are: software developed and designed by engineers.