In a word: Nothing.
My iPhone 4S was stolen a month ago after I had barely had it for a year. It had everything installed including "find my iPhone" and iCloud, etc.... what Apple does not tell you is that these location services are useless if you are dealing with experienced thieves who know exactly what to do... Despite Apple being technically able to deactivate the phone instantly (provided you can give them a police report number and the phone's serial number) they REFUSE to do so and claim they "have no program" for such an event.
Oh and btw: LAPD classifies the theft of an iPhone as petty theft and as such will not do a thing about it - 'we don't do iPhones" I was told...
All your phone service provider is generally willing to do is interrupt service but not much else. What does it help you to know where the phone is via GPS?