as far as the RFID proximity sensor like nissan uses.
these are very expensive and fairly complex. it does have a good write up and project in RFID toys on these types of systems but it is not for the faint at heart.
active RFID systems like this require custom code as well to integrate with other systems. it can be done but its not going to be easy.
I have used many of the projects as starting points for projects for work (network/systems administrator). specifically to control our access gates and secure areas door locks.
the book provides very detailed step by step instructions that explains why you are doing certain things as well. this makes it easy to take a little bit from each project to develop your own systems to meet your needs.
one thing i have considered doing is using an rfid system in the car for this purpose that would act very much like nissans proximity sensor.
explanation:
have an rfid reader in the door as well as the center console or seat.
keep rfid card in your wallet
when your wallet gets close to the door less than a foot away the door unlocks.
when you sit in the seat the rfid card is read and triggers the car to the start position.
press the start button and away you go.
this system would cost about $200-$300 bucks but would require some real engineering to pull off...
I would be willing to put the time in to it if I had the time....
how cool would it be to not need to carry any keys any more.
you could rig the trunk the same way and the door to your house (doors are extremely easy and cheap to do if you have 110v near by)
I might have to do the research the more I think about this I want to do it..... it would be awesome