Just put an ADT or Altronics sensor on there. Then when you forget to disarm it, and the alarm goes off, and the cops come by, well..... let's just say, you won't forget to disarm the alarm the next time....
I would think this is something that would be possible. The only problem I see is that pickup beds are used for dirty loads sometimes, like hauling dirt, lumber, and other stuff that can bang around back there. I would think this could make the sensors susceptible to damge quite easily if you haul more than azz in your truck.