The medical news services have been running with this story, a study of a large database in California which looked at the effect of ambulance diversion, finding an association with increased MI mortality.
A few caveats. The association only held up for the more extreme scenario of 12 or more hours of diversion. In this era of reperfusion, AMI is one of the most time sensitive conditions presenting to emergency departments. So no conclusions can be drawn about ambulance diversion overall. For other populations of patients it is not known whether the risk of diversion outweighs the risk of hospital crowding and ED or hallway boarding.