This exact scenario happened to one of our moderators, I'm sure he will jump in if he wants to share it again.
Dial 911, grab my Social Occasions gun, get behind whatever cover I have availible and let him know that he needs to get the hell out of my truck and lie on the ground with his hands behind his head.
If my wife was home, I'm sure that she would also arm herself and seek cover.
Responses from that point depend on the actions of the perp.
For people who say that they would just leave him alone because mere possesions are not worth getting involved, Screw That! I spend most of my life working at a job that I don't really enjoy so I can have money to buy stuff like that. When some scumbag is taking things from me, he is taking hours and hours of my life away from me!
Thanks to ctdonath for articulating that argument in a earlier post.