Would that mean that the cake is not, in fact, a lie?
Or is it a deeper commentary on the realism of a representation of a thing, a la Magritte's The Treachery of Images? The cake is a lie because it's not a cake, it's a representation of a cake.
I was watching the screensaver of a Big Buck Hunter arcade cabinet at the bar last night, and one of the minigames involved shooting roaches crawling all over a house. The cake in the kitchen was definitely the Portal cake.