Via Business Traveller, China has just announced that as of January 1, 2013, it will be possible to have a 72-hour transit in Beijing without a visa for residents of 45 countries (including the US). The article does say that tourists will have to register with their visa at a police station within 24 hours, though I’m not sure how strictly that’s enforced.
Now, for what it’s worth I don’t think this will work for those doing mileage runs with direct turns to the United States. Shanghai currently offers a 48 visa-free transit, though you have to be continuing onwards to a third country. After all, if you’re arriving from the US and continuing straight back to the US you’re not really in transit. Though I’d love to be wrong, as that would be great news for mileage runners.