I your car is parked near your network and it is broken into the empeg (with appropriate software and external interface board) could send a message to your PC. The PC could then do some of the following:

email you
fax you
yell at you (through the sound card)
call you
page you
call the police
use X10 to:
blink some lights (inside and/or outside the house)
turn on the lawn sprinkler system
turn on (or off) the TV to get your attention
make coffee

OK I'm sure you get the idea. Car alarm sends message to PC who in turn reacts in some amazing and spectacular way to help you save the car (and empeg), or at least run out and yell at the kids who set it off by accident while playing ball in the street.

Later,

Steve