AH! Then it is probably a wheel out of balance. I'd suggest you find a shop on your own and have them road force balanced as some others have suggested. Would cost you $100 or so, but it'll be done...
If you can still return the car...sadly return it and explain that they and you have heard the problem and it has not been fixed. So, regrettably you're returning it and want your money back. You might point out you might be willing to consider repurchasing it if they contact you after the noise is gone. As much as you love the car it will never make you happy if you spend months searching for a repair to your "new" car. I suspect the dealer will figure out how to fix it rather than lose a sale in the hand rather than waiting for one in the bush. If not there are many more Prii out there !
Ok, so update. Today I told my husband to take it back and tell them the sound is getting gradually louder. It is NOT the tire and that he insist they look and it and potentially replace the tire. After TWO hours, they came back out to my husband and told him that they wanted to test drive one of their lot prius and see if they made the noise. They told them the noise was evident in the other prius but VERY faint and quiet, they had to listen hard to hear it. They think it is something with the generator and they have to call Toyota about what to do. They said it's the generator that generates power into electricity from the wheels. I'm SO confused and I don't know what to do.. They are apparently going to get back to us once Toyota responds to their inquiry. Fully covered under warranty so I don't need to worry about $$... it's just annoying as shit that ALL this time they have been telling us to go away and 'enjoy our prius and that they make funny noises'... I can deal with the weird egg timer noise on the brake accumulator -- but this rolling intermittent "vrrrrrp.....vrrrrppppp......vrrppppp" is making me frickin nuts!
All I can tell you is mine doesn't make such a noise, and its 7 years old with almost 80,000 miles. Power is only being generated when the regenerative brakes are applied, when the car is rolling...its just like any other car. They're full of shit.