Another factor could be that refineries are going to have to invest to be able to meet the 2006 US requirement for lower sulfur levels in Diesel and they need to get everybody used to Diesel prices higher than gas.
Plus it's likely my fault as we just replaced two gasoline vehicles with Diesels. Demand for gas fell and demand for Diesel rose.
In some parts of the country, ULSD is already available. I've been running my car on it for a couple months. It has additives to address the lubricity issue.