Got the same message as others @1:46 pm, 3/21/16. "officer Mark Watson of the IRS claiming that there is an arrest warrant against me and that my physical address is under investigation." I had similar messages (5) about a month ago.
Talked to them for a while. They're getting smarter, asking you to honk your car horn to prove you're in your car, and asking for the specific moneygram phone number on the send money form.
The "Senior Officer" confirmed to me that they were in Islamabad, Pakistan. For some reason he seemed to think that the Miami number he was calling from is the IRS direct line...