Initial USGS map centers it south of Pomona at 5.8 magnitude. (Expect that to change in the coming hours and days.) Here are the geographic coordinates for the epicenter:
3 km (2 miles) SW (235°) from Chino Hills, CA
8 km (5 miles) SE (127°) from Diamond Bar, CA
9 km (5 miles) NNE (23°) from Yorba Linda, CA
11 km (7 miles) S (178°) from Pomona, CA
47 km (29 miles) ESE (103°) from Los Angeles Civic Center, CA
Time was 11:42 am.
Noted: I received a half-dozen emails saying that LATimes.com went to "server unavailable" status minutes after the earthquake. It's back up now. Some cell phone service is out or overloaded, but there are no reports of major damage. [High traffic was to blame, say editors.]
12:35 update: Already downgraded to 5.4 magnitude. The coordinates have also been adjusted slightly. Major damage is not expected, but that won't stop it from being all over TV and the Web all afternoon. Here's an initial shake map from USGS; when it ain't red, be happy.