Government found liable
here's a report of a verdict and settlement over a 911 call. The woman was being beaten, and ultimately smothered, while she made repeated 911 calls. For some reason the dispatcher merely sent out a call relating a domestic disturbance. The officers took 26 minutes to respond, found nobody responded to the door, and naturally left (you don't bust down a door over a report of a "disturbance," esp. when all seems peaceful).
As a general rule a person cannot sue for a failure to protect. The story refers to a finding of recklessness, which suggests to me that in this state you can sue for a reckless or willful failure.
Hat tip to Curtis Lowe and his blog.