Great designers are in demand. Time is their most valuable asset, so in order to get a piece of this asset they must be offered a form of compensation. They clearly do not need to be designing for "would look great on your portfolio" nor they are likely to be enticed by the "gratitude of the masses". The only non-monetary compensation that works is to make them genuinely interested in helping -- and that if you think about it is the exact reason why devs give free time to open source projects. If it's not interesting, they leave.

So as harsh as it may sound the answer to your question is that you just cannot get great designers. There's virtually nothing you can offer them in return. If you do manage to get someone on board, do shield them from the "community". Work one on one and put all your weight behind his designs when they are made public. Leet hackorz that are so vocal on many O/S forums and mailing lists can easily demotivate even Mother Teresa in a matter of few replies.

HTH, feel free to vote me down now :)

(edit) On a second thought VLC might be one of few projects that can attract a designer because of its high-visibility nature. Yet the point remains - designers are easily demotivated, way easier than developers, so retaining them is a major challenge.