Working on something like this for my boat. A guy on one of the other forums I hang out at is selling a push button starter that replaces your key.
I am going to add an RFID system to his set up when I put it in my boat. I'd been looking at a commercial solution for my motorcycle for a while, but just couldn't pull the trigger on $175. Bought three of the RFID transponder setups off of Ebay for $18. I thought the range of the RFID would be better but it's only advertised as 3"-4" from the antenna on the Ebay units I bought.