An mysterious story from Associated Press, pick up by a number of major newspapers, new research is announced on ambulance diversions and overcrowded emergency departments. The study's lead author, Catharine Burt, of the National Center for Health Statistics, used data from 405 U.S. hospital emergency departments – about 10 percent of the nation's hospital ERs. This Modern Healthcare story has some additional data.

Using research from the 2003 National Hospital Ambulatory Medical Care
Survey, the Centers for Disease Control and Prevention found that
hospital emergency departments diverted ambulances when they were
overcrowded, citing a lack of appropriate inpatient beds (51%), a high
number of emergency department visits (50%), and complexity of
emergency department cases (18%).

A separate study by UCLA researchers, published with the CDC's findings online in the Annals of Emergency Medicine, found that ambulance diversions at Los Angeles County hospitals more than tripled between 1998 and 2004.

Sorry, no links to the actual paper – allegedly in the Annals of Emergency Medicine – there's nothing about the paper on their site.