"Bitcoin, which has been getting a large amount of public attention over the last year, represents a radical new approach to monetary systems which has appeared in policy discussions and in the popular press. Its cryptographic fundamentals have largely held up even as its usage has become increasingly widespread.The key issue is that nodes on the network have an incentive to keep transaction information to themselves to improve their odds of earning the transaction fee. These researchers propose a system to fix the incentive structure.
We ﬁnd, however, that it exhibits a fundamental problem of a diﬀerent nature, based on how its incentives are structured. We propose a modiﬁcation to the protocol that can ﬁx this problem.
Bitcoin relies on a peer-to-peer network to track transactions that are performed with the currency. For this purpose, every transaction a node learns about should be transmitted to its neighbors in the network. As the protocol is currently deﬁned and implemented, it does not provide an incentive for nodes to broadcast transactions they are aware of. In fact, it provides a strong incentive not to do so. Our solution is to augment the protocol with a scheme that rewards information propagation."
Edit: here is a simplified summary of the paper.