Later on in the game, there is graffiti on the wall stating “The cake is a lie.”
And at the end of the game, GLaDOS sings the song “Still Alive” (written by the ever-awesome Jonathan Coulton which, as you expect, discusses the cake some more, as the image of the cake (with a lone candle) sitting oh so lonely in a storage area, appears.
The lyrics are available here, or you can listen to Mr. Coulton (and several hundred Portlanders) sing it in this video.
Why did I create this page? Well, I wanted to explain to someone why the cake is a lie, but I couldn’t find any short explanation on the Internet that didn’t sidetrack into heel springs and weighted companion cubes, so I made one.