one other tactic that is prominent with promotions like this is just to get you into the showroom. Look at the G8, drive the G8 and thats if you can since there will most likely be line waiting to do the same.
Then the idea is if you cant afford or dont like the G8 is to show you something that you can afford...many dealers have tons of leftover models they want moved badly. The slumping economy is hitting them hard as well. One of my co-workers had his daughter go to one of these things at a Dodge dealer and she came home with an 07 Charger R/T just because they discounted $7000...granted thats a big chunk but the car was still over $25k and that doesnt make that Hemi get any better gas mileage.