1) In the graphic design world, these three letters are usually written large (and possibly in red letters to call attention to them) on a photo, illustration etc. that will be replaced with the actual one at a later time. Used in creating graphic design concepts when the designer does not have all of the artwork yet.
2) An object, possession, or anything else that will eventually be replaced with a nicer version very soon.
1) John: "Dude...I didn't know that Evangeline Lilly was the spokesperson for that new Kia Sedona ad you're working on."
Evan: "Down boy. I just downloaded it off the internets...it's FPO. We have the actual photo shoot this Friday with the real model. They can't afford Kate from Lost.
John: "Oh. Right."
2) Diann: "Did I see you driving your mom's 10 year old mini-van?"
Carrie: "Yeah. But it's FPO. I pick up my new Jetta tomorrow.
Diann: "Sweet!"