Thanks to everyone who replied! I've been flying Southwest a lot lately, so that's where I've been hearing it. I believe the announcement is made by a FA, not by the pilot, but I could be wrong.
LarryJ, I appreciated your post indicating that the SWA FAs use that phrase to tell the pilots that the passenger cabin is ready for takeoff. I had always assumed the reverse, that it was the FAs telling the pax that the plane is ready for takeoff.