sigalabs.com have designed and build a great OBD2 Arduino Shield using STN110 IC :
The STN110 is a multiprotocol OBD to UART interpreter IC. It provides an easy means of accessing vehicle data, including diagnostic trouble codes, MIL status, VIN, Inspection and Maintenance (I/M) information, In-use Performance Tracking (IPT), and hundreds of real-time parameters.
It is compatible with famous ELM327 commands but introduce many new great features.
Vehicle OBD2 Arduino Shield with STN1110 IC – [Link]