That "eco-credit" $4500 is the same $4500 BMWNA was offering dealers as trunk money back in June-July, now rebadged as green money.
It's $4500 off of whatever price you negotiate with the dealer, MSRP, invoice plus X or what ever. Up to the dealer if he wants to give it to you.
If not, I'd walk.
Dealer sells the car, BMWNA pays the dealer $4500 (plus an incentive percentage - holdback, basically - if the dealership qualifies).
It's free PR for BMW for money they were giving away anyway.