The last time I was there, things had changed a bit;not sure if it's due to Nationals economic problems or if it was 9/11 related since we were there in November and travel was really slow then, but....
... you could reserve your car at the Dolphin, but, the National shuttle came and got you and took you to the car care center where you picked up your car. Return was same thing in reverse.....you left your car at the car care center and shuttled to the Dolphin.
There were a few people who complained about this change and not being notified,and they were allowed to leave their cars at the Dolphin and somebody from National came over there and got it.
I would definitely call and ask before hand.