I arrived back in NY yesterday from London and I've not traveled inbound to the US since February. No paper I-94 (white arrival form for those traveling on US visas) available at check in or on board. One of the pursers found me a spare form from his bag.
The crew on the flight did not appear to know why, and no PA was made, but I was told by the immigration official that these forms are no longer needed - they have been replaced by an electronic record (as the I94W green forms were a few years ago).
You can get your admission number from a CBP website:
http://www.cbp.gov/i94
This could perhaps be worked into the onboard 'customs form' PA for the next few months?
To be fair to the crew all we were told was they had been removed from the aircraft, no further explanation given. It was only through my own research that I discovered they were electronic.