I had a 58 plate from November till first week of Jan then swapped for my private plate. I appreciate the coudos of having a new 09 plate but quite frankly I have had as much attention with the new plate on as the car is still very rare.
Having to register a new car on your private plate also means more form filling and a wait from the DVLA. By waiting and replating a registered car it only took four days to get the OK through on mine.
JPZ, get it registered on your private number, saves so much hassle as the dealer will sort everything out for you (although i'm not sure if your transfering from your bike). Mine is on retention but i didn't get the certificate in time to register the car on that plate so i have to sort it myself. At the end of the day, there are so few rocs about everyone will know it's brand spanking new