I (a US citizen) recently returned from a trip abroad. When I entered the US via San Francisco (SFO), I had to do a bunch of stuff at a machine: scan my passport, answer a few questions, and take my picture. Then I received a receipt from the machine with a bunch of info on it.
When I walked up to the US Customs Officer, he stamped my receipt, but not my passport. When I got my luggage and passed through the last part of customs, the stamped receipt was taken.
What's the deal? In the past, I would usually get an ADMITTED stamp in my passport.