At the port of entry, the INS rep must validate the I-94 form - Record of Departure/Arrival - which notes the length of stay for admission in the US.

I believe that once the visitor leaves the US, that I-94 form is removed from his passport and returned to the INS office ( don't know whether this is done by the Airline or security before boarding international flights from the US)

As a Canadian, I am used to transborder flight and I don't know exactly who removes the I-94 from the passport but I have a relative who is on a G-visa in DC. She travels a lot and she goes through that process of getting the I-94 form every time she goes through a port of entry. During her last trip back to the US, she came through IAD late during the night and the INS officer didn't include the I-94 in her passport and she has been requested to go back to Dulles with proof of that recent flight so that they can issue another I-94 form and the proper authorization to enter the States ( believe that since she is canadian, the INS officer didn't think that she needed one). Otherwise she may be in trouble the next time she tries to leave the US for another overseas trip