To return to something we discussed in the IRC meetup: there's a simple argument why commonly-known rationalists with common priors cannot offer each other deals in a zero-sum game. The strategy "offer the deal iff you have evidence of at least strength X saying the deal benefits you" is defeated by all strategies of the form "accept the deal iff you have evidence of at least strength Y > X saying the deal benefits you", so never offering and never accepting if offered should be the only equilibrium.

This is completely off-topic unless anyone thinks it would make an interesting top-level post.

ETA: oops, sorry, this of course assumes independent evidence; I think it can probably be fixed?