Yes--I, too, have often heard the FAs announce that those in bulkhead seats must use the overheads. To my knowledge, I have never been on an airline that required you to use the space under your own seat, it has always been "the seat in front of you."
I just wish that they'd make these announcements every time. I have had folk try many times to put their stuff in the space occupied by my feet and it does get annoying.