The last two times I've gone through the border since the new rules went into effect in June, I just gave the border guard my DL and birth certificate. First guy didn't say anything, 2nd guy made a joke about me having to go back through Nogales and that I was now a Mexican citizen. Hah, very funny. Jerk. He gave me a notice, the other one didn't.
Still haven't bothered to get my passport, but may do the application at the last minute through the post office & show 'em the receipt.
Legally, they can't retain you if you can prove beyond reasonable doubt that you are an American citizen. (There's something about 24 hours and a consulate visit in the law, sorry I don't have a link now.)
Now, keep in mind I look like a stereotypical clean-cut, white, blondish hair, green eyes.. If I was ethnic looking in any respect I wouldn't mess with not having everything in order.
So, bring the old one & a receipt for the new one! (Btw, what exactly do you mean by "old" passport? Expired? Otherwise, you can still use the old book format.)