That stuff I wrote about the internal protection diode, Vin shorted to ground, 15A of surge current and 50uF came straight from the datasheet for the part. It recommends an external diode if you are planning to exceed those limits. Since it has an internal diode, it's pretty much impossible to reverse bias the device.
Ok, I found the other "USBVCC" on the schematic that flows thru the P-channel FET. I really dislike disjointed schematics.