timing for tax credit

I have been on a wait list since September for an 06 silver pkg 5. I was geting a little tired of waiting so I just called another local dealer and found out that they have an 06 silver pkg 5 coming in on 12/12/05 in New York that is available. It will be delivered to to dealership probably by 12/18th. To get the tax credit do I need to wait to pay for the car after 1/1/06 or is it ok to register the car after 1/1/06 and pay for the car in December? The dealership did not know the answer to this question. And I probably would not trust their advice.