As we mentioned in our write-up on GM's revised Duramax, the company showed journalists a prototype light-truck implementation of urea injection. Such technology allows for the reduction of NOx emissions, but the EPA has been hesitant to approve it. GM may be moving forward with it anyways, as 2009 emission rules for light trucks require that diesels burn as cleanly as gasoline engines. The alternative is a particulate trap, which hasn't yet demonstrated the required durability. Urea injection requires an occasional refill, but I'd personally be happy to take on an additional maintenance task in exchange for a vast improvement in fuel economy.