Arduino interfacing with Home Alarm System

Hello Everyone,
Just wanting to introduce myself, say hello and ask a few questions. Back in the day, I helped my Dad assemble a few kits from Knight and Heathkit. I still have a multimeter and tube tester we assembled (not for use). From an old Forest Mims booklet, I've built two 555 monostable timers pulse generators for a stepper controller driving a platform for wide field astrophotography. No knowledge with microcontrollers. I have good soldering skills and basic electronic knowledge.
What brings me to this forum is a problem that requires a remedy. My neighborhood has always been a quiet and safe place to live. All that has changed in the last 7-8 months; there has been a rash of home burglaries, hitting 7 of the nearby 20 houses. I have a friend whose wife is a police detective. She tells me the mo is a group of people will simply drive around a neighborhood and pick a house to rob. The driver will drop the others off in front of the house, either stay parked or circle the block and return. While the driver is doing this, the others will force the door, hit the garage door opener, the driver backs the car into the garage closing the garage door behind. The burglars quickly load the car in the closed garage. If there is no garage, the burglars will quickly gather the major $$$ items near the front door, after the driver returns from circling the block, quickly load the car and off they go. All this happens in less than five minutes. She thinks it is one group hitting my neighborhood.
If the home has an alarm, the control panel is found and smashed to quite the alarm. If the home alarm is monitored, the police do not response in time to do anything other than file a report. If there is an outside disconnect for home's electrical service; the power is cut. With battery back-up this should not do much other than ruin what is in the refrigerator. The burglars cut the power during one robbery of a nearby home.
So that is my problem.
Since a monitored alarm is not much use, I have canceled my monitoring service. What I would like to do is make my house a not-so-good place to be if the alarm is activated. The plan is to install several high dB sirens inside and outside the house.So that is part of my solution (hopefully a good one) to my problem. The sticky part, for me at least, is that my existing alarm panel has a circuit for external sirens. When the alarm is armed or disarmed with a key-fob device the external siren circuit is energized for about 1.0 to 1.5 seconds. My wife uses a key-fob. Before I found out about the external siren circuit energizing, I planned on using the external siren circuit with a debounce circuit to trigger a 555 monostable circuit. This circuit would be remote from the main panel so if the main panel was smashed the 555 with the extra sirens on a relay would scream on. So no go on the simple 555 circuit.
My questions:
? With a proper power supply, is the Arduino a stable and reliable platform for my application?
? The planed power supply is a power brick from a scrapped LCD monitor; it is a switched power supply, 6 ampere at 12 vdc. Should the Arduino power supply be separate from the siren power?
? How sensitive is the Arduino to power spikes? I plan on using a surge protector on the power brick.
? Looking to add automatic power transfer with battery back-up at a later date. I guess that is not a question, just throwing it out there.
Not asking to be spoon-fed, but I really would appreciate any ideas, critiques or anything helpful. I have just ordered the Duemilanove. I thinking my main problem is going to be writing the script.
Thanks for reading, sorry so long.

Hey Richard,
Thanks for the reply.
The Arduino will be remote with an independent power supply on a different 120v branch circuit. Automatic power transfer will be the final phase and many take me a bit of time to figure out. Getting the sirens installed and on a seperate relay and supply is the first order of business.
All the burglaries are during daylight hours while the homeowners are at work. The detective told me they just go about their business in a matter-of-fact way. No behind bushes or anything to conceal or hide, just step out of the car stroll up to the door and go about it. No tires squalling or anything that makes them stick out. If the neighbors are not looking close, they may just think the plumber made a call.
With standard time here again, the motion detectors will be going in this weekend.
Thanks again,