From Jim Hamilton who has a great piece on why gas prices differ across states. The main cause is differential taxes, but there is a lot more going on - go to his blog to read the post. Here is a map from his post that shows gas taxes by state:
One thing he fails to mention is the ban on self-service gas in OR and NJ. But as a rough guesstimate were are probably talking about 3 cents a gallon, which pales in comparison to the taxes above.