The following article just came out in Science. Brooks King-Casas, Damon Tomlin, Cedric Anen, Colin F. Camerer, Steven R. Quartz, P. Read Montague, “Getting to Know You: Reputation and Trust in a Two-Person Economic Exchange,” SCIENCE, 308, 2005, pp. 78-83. This is a really interesting paper which shows the power of Read Montagues efforts to put together a hyperscanning community. The paper implements a scanner friendly version of the Berg et. al., "Investment/Trust Game", so as to simultaneously scan both subjects acting as trustor and trustee.
One of their main findings is that the head of the caudate nucleus is active as the trustee learns to reciprocate with the trustor. At first this activation is observed at the time decisions are revealed, but later on the activation shifts forward in time in anticipation of receiving trust from another.
The same phenomena is observed in reward processing areas of the brain associated with dopamine neurons such as the nucleus accumbens, which has been hypothesized, see the review by McClure et. al., to be a form of error-prediction learning in the brain. This suggests that at first people closely monitor each other and use costly brain resources to invoke a "theory-of-mind" model of each others intentions, as seen in the earlier work by McCabe et. al. But over time the brain economizes on scarce resources by coding an anticipatory response to the trust arrangement.