Everyone has now played Portal and knows about the trials and tribulations of the Weighted Companion Cube. Even those they haven't played the game have seen the cube in many iterations (cake, ice, papercraft) around the internet. It's another Valve sensation and why shouldn't they capitalize off that?
According to the Steam website official plush version of the companion cubes will be coming in two flavors; regular plush and fuzzy rearview-mirror dice.