The first attempt at this project was using the ELM327 Bluetooth dongle.
But I came to find that the available OBD info provided by the dongle was insufficient. It only provided speed, RPM, water temperature and a couple other sensor readings.
But nothing relative to fuel consumption...
Then I switched to another approach: "what if I could drive the OBD bus directly?"
So I searched the web for OBD implementations using arduinos and found this web page (OBD2_KW1281) that described very well the KW1281 protocol (used by my Audi A4) and had the code for arduino too! Nice!
I'm using STM32 with the arduino IDE gathering the best of three worlds: good performance, low price and vast libraries/code available. You can check it out at www.stm32duino.com.