YES!!! It can be done. Im an auto tech and there are a few ways this can be done. telematics automotive automation. Im working on this now with the n900. First:

1) relays must be used injunction with the ignition system

2) when it comes down to control you have these options

a. arduino to control the relays via N900 wifi, bluetooth

b. simentry gsm module. check out website www.sim-entry.com this will activate - deactivate the alarm system

the hardest part of this would be that a program must be written to communicate between the n900 and arduino the simentry module is simply controlled by a sms from n900 which is native to the phone.