Mark Whittington has a proposal for an asteroid prize. The prize suggestion is on page 3 of the article.
One problem with the current implementation of the Vision for Space Exploration, NASA's ESAS architecture, is that it doesn't plan to return exploration results until 2020. That's a long time for taxpayers and politicians to wait. If a human asteroid mission done with, if we must, the traditional contracting approach can be done before a comparable lunar mission, it's tempting to hope for that route, followed by either Mars or lunar missions. However, I suspect that an asteroid mission using that approach would also involve a huge time gap between the decision to do it and any actual results.
However, the proposal that was publicized before the Stanford meeting Mark describes actually didn't suggest an asteroid as the next step. It suggested adding the capability to an Ares I/Orion baseline, using commercial assistance like a Bigelow module, to do Lagrange point satellite servicing missions to maintain multiple assets like the James Webb Space Telescope. This sounds like it could be achieved much sooner, in which case it would be a quite interesting stepping stone whether the next step is an asteroid or the Moon. It wouldn't solve all of the current problems with ESAS, but it could be an improvement over current plans. Perhaps it would keep Ares 1 productively busy before Ares V is built, allowing commercial vendors to supply the ISS without political resistance from Ares 1 operational supporters.