AFAIK (correct me if I'm wrong) you no longer routinely present your passport to CBP at US airports, but merely look into a camera and have your details pulled up from the flight manifest. Logically, then, passports aren't stamped.
What cohorts, however, remain required to have their passports stamped?
I'd ask the CBP, but the Info Center is useless and I don't have the email address to the Traveler Policy Division which I suspect is the responsible department.