I've heard (and used) this when I've traveled in uniform, but usually the announcement is just for those in uniform. At AA it's common for every flight, and when I flew UA about to the deployment I'm on now, the same announcement was made as well for both flights. Not as common with Delta (not that I care), although on one DL flight I was on, there were a handful of military personnel at the gate area in uniform and all other UGs had cleared so the GA UGed them as well.

EDITED TO ADD: What I have also heard a couple times as well (on DL and other airlines) is when the plane parks at the gate, the FA will ask all the other passengers to remain seated and allow the one or two military in uniform on the plane to get off first. I'm not going to ask or even expect it when I fly home, but if it does happen, I'll appreciate that more than getting on the plane first.