Two different questions. A U.S. passport is a valid travel document until 23:59:59 on the printed expiration date. But a U.S. passport is usable for the purpose of entering the United States even after it's expired because it remains valid proof of U.S. citizenship even after it's no longer a valid travel document.

(Incidentally, countries' willingness to accept their citizens back on expired passports is the reason the 3- or 6-month validity requirement is waived in some cases)

CBP may send you to secondary just to make sure you're not an impostor who got his hands on Mr. X's expired passport, but a traveller who arrives at a port of entry with his own expired U.S. passport will be admitted with relatively little hassle. GUWonder is correct--if you're flying, the real issue is convincing the airline to let you board.