My article in the September 2007 Two Plus Two Internet Magazine is on NFL Power Ratings. Here is the link to the article: NFL Power Ratings article (the link will only be good through the end of September).
A poster by the handle of SumZero made a good post with a better formula compared to the simple addition/subtraction. Here is his post: SumZero's post If you read the article, I suggest you read his post as well.