I know that since most cars use a nominally 12v electrical system, theoretically it should be possible just to route this into the Arduino's power plug. However, the "Practical Arduino" book notes that the actual voltage in auto electrical systems can vary much more widely and suggests using a separate power supply with its own regulator. How are you handling this? Thanks...
An auto battery systems voltage can vary quite a lot between depending on state of charge, etc. I think 16vdc is the max one can expect to see. This is not too much for the arduino external power although the closer to 8 volts the cooler the regulator on the arduino will run.
The real problem with auto voltage is all the noise and load switching going on can cause lots of problems with some electronics. There is also a phenomenon called a load dump where almost 50volt can be spiked onto the auto's 12volt system. Anyway lots of people suggest to utilize extra filters, transient suppression and over voltage protection for sensitive electronic used in autos. The input filter capacitor on the arduino's voltage regulator is probably the most vulnerable component and I forget it's voltage rating.
Lefty