GPS, as everyone knows, most phones now have a gps chip in them. It is possible for your carrier to track this but I don't know if they are ever going to be willing to track it though. Also, put out a reward for your bag, let people know.
The easiest thing to do is to go put insurance on your plan BEFORE CALLING YOUR CARRIER ABOUT IT BEING LOST and then a few weeks later call and file a claim, pay your deductible, and then get your new phone. Then again, it's always easy for me since my bro's roommate works for AT&T