"Find My iPhone" is a great way for you to see where the person who stole your phone is using your phone, and then realize that there's nothing you can do about it. "Yes, hello officer. My phone is at the Burger King on Broadway. Can you search everyone there and see if they have my phone?" The only real use for this iCloud app is locating your phone after it has fell between your couch cushions.
Your girlfriend might have a use for the app though. If you've ever used your iTunes account on your her phone to download a song, she might have access to your iCloud account. When you leave her spot and go see Christine, she'll watch your no-good-cheating ass, represented as a blue orb, travel to Christine's crib in real time.
It will soon be time to "Find a New Relationship."