This is a nice trick on thieves. However, just like a car alarm; people become "immune" to noises. Might be nice to have a voice recording such as: "Stop. Thief. This iPhone is being stolen!" Also, some ideas for use is to put it in a conspicuous place in the house so if someone does enter, it may be the first thing they go for and will alert you to a break-in. Ive been broken-in twice so this would be another deterant, these folks move through quickly so it may just work!
RudiH about Thief Alert : Burglar Alarm