http://www.sciencemag.org/cgi/content/a ... 9/5989/325
And for the German speakers more details here:
http://www.heise.de/tp/r4/artikel/32/32968/1.html
Basically what they found out is that the Pay-What-You-Want model (as seen recently often in the indi / linux gaming world) works even better if you promise to share the profit with a charity. You get more cash and the charity is an added bonus. Their explanation is that the charity aspect lowers the amount of free riders, who pay only minimal amounts.
Sound interesting maybe also for OS game projects with commercial aspirations.
I have the feeling though that the Pay-What-You-Want model loses it's strength once the novelty is over.