Personalized plates stolen, need some help

I dont know how well this would work in your particular application, but heres a shot...

Mount the plate using nuts and bolts (assuming you are using std. bolts w/the nylon inserts (?).  Leave the nylons there  - just get something that will go through it and come out the back.  About 1.5-2" long.  Get (4) bolts, (8) washers, (4) nuts, and (4) springs from your local hardware store, and as many (sealed weatherproof) pin switches (NC) as you like. 

Put the bolts through the plate, then a washer, then spring, then washer, then push through the nylon insert and set it about .5" off from a flat surface behind the lic. plate.  Figure out the area you have to work with (behind the plate) and setup a (or many) switches behind the lic. plate.  Set the action on the switch(es) so that the head of the pin is barely making contact with the back of the plate. 

Once you have figured out all the necessary depths etc. you want to put the plate fasteners back on (all 4 of em) and once the depth is set  properly - put the bolts onto the back of the plate (and add some red loctite for good measure).  Connect the output of the switches to an input on your alarm and voila.. 

If this works as I suspect it to - youll have a theif that puts the screwdriver up to the bolt and upon pressing the bit into it - will depress the plate - thereby depressing the switch(es) and the alarm takes it from there. 

I can't say I have used this for license plate protection but I have done some vehicles in the past that have had seperate (alarm) zones for the vehicle, and for the (stereo/ice) components inside.  The pinswitches would attach to "floating" amp racks, backs of LCD's, etc. etc. and if the customers were @ shows or whatever - they could disarm the primary zones and leave the component zones hot.  It worked pretty good for that sorta thing...you just may have some problems with temperature/humidity and its effects on the switch itself.  I've used some from Farnell electronics before that were end switches for process controllers...they seemed to do the trick pretty good.  Then again, we're talking about a couple hundred bucks to save 75$ (?) plates...so its entirely up to you. 

Good luck!