UPDATE: Turns out it was a 5.8 a few miles east of Diamond Bar. That's the most shaking I've felt in quite a while, though.
UPDATE UPDATE: I guess I should report that everyone here is fine, and there wasn't any damage. This is going to turn out to be not much of a news story, but it sure felt like something. It went on a while; I'm hearing 20-30 seconds, but reports are all over the place right now. Every local news outlet, and then some, are talking to anyone who will tell their "this is what I was doing when it happened" story.
UPDATE^3: Reports have now downgraded the magnitude to 5.4.