If you can't trust a car salesman, who can you trust?

A local Dodge dealership has been running ads for $15,000 cash back on new trucks. They make it a point to point out that it does not get added on to the price of the vehicle (i.e. you don't buy a $30k truck and finance $45k to get the cash, I'm assuming). So what's the catch? How is it possible to get a $30k truck for $15k? I'd love for someone to explain the slight-of-hand that my cynicism dictates must be taking place here...