All the other products and places seen on billboards in the game can be found, but not Rita's Café.

Is it the new Giddy-up Buttercup?

Might appear in a future DLC, although I recall some posters in Fallout 3 that advertised products and shops that didn't appear in the game (such as the halloween costume shop)

The costume shop was in Paradise Falls, if I remember right.

But yeah, most likely DLC.

My money is on it being DLC. And I think you could find a giddy-up buttercup somewhere (decoration of course), and there was a kid (in rivet city maybe) who asked about one.