What electrical contact material is used for PF Connectors?
The material is most likely aluminum or tinned copper, here is why:

Most LEGOs are made in Denmark, the country of origin, with other various quantities made in mexico, Taiwan, and other factories around the world. Denmark is a huge exporter of electrical generators (see wikipedia). Making generators requires a good amount of aluminum and copper to make, especially if the generator has a build in power transmission. I would be willing to bet that the same materials that are available to make generators would also be available to make LEGOs, copper and aluminum. I'm leaning towards tinned copper because of the luster in the pictures, but I couldn't tell you for sure, and LEGO doesn't provide a datasheet.