Passport Stamp
It's at the discretion of the person sitting at the desk. Sometimes it happens, sometimes it doesn't. It's not just at FCO. I've had it happen many places, even driving across European borders in the 90's before the EU was founded. When the agent at Belgium didn't stamp my passport or my children's' I complained, and he told me it was not necessary, and then he grudgingly stamped them. I'd say that nowadays, going to the EU 5-6 X's per year, stamping is about 80% regardless of where I land from the U.S. Just ask if you want it.