Thanks for that information!
Those three things that are cascaded are transistors for the relays, not reg's.
I've used this simple design for my relay setup Arduino Playground - HomePage
Well, the way i've currently wired it, the arduino and relay coils get their power straight from the battery, and the arduino has a 6v reg to step down from 12v, the bloke at jaycar said that the reg has inbuilt thermal and reverse polarity protection and should be sufficiant to protect the arduino from abnormalitys.
I've used a very expensive multimeter, which has a .01 sec refresh rate and also logs. i had it on the car battery whilst starting it, and the highest it jumped to was 14.8v half a second after starting. then back down to around 13v.
The only other option i was thinking of doing was having the arduino's power supply running from a seperate li-po battery, as i have plenty of these, could hook up a recharger that recharges the li-po's when the car's running.
thus eliminating the arduino's power supply completly from the cars supply.
As far as having a normally closed relay between the arduino's power supply and the "ON" key wire in the car, i thought this would be adequet enough and 100% safe as it kills the arduino's power completly, and as i mentioned in my first post, i'm sort of new to this field, so i thought this would be the best start point, i suppose eventually i'll move up to a state machine.
As for the fuse for the arduino's power supply, what's the rating i should use?