The cake isn't a lie, after GLaDOS is done singing, it cuts to the Weighted Companion Cube in a room with other Weighted Companion Cubes, and a chocolate cake... Plus, after you beat the game, the main screen is playing that song with more cake...
But either way, I like this quote: "The cake may be a lie, but the pie is moist and delicious."
Well, the cake is a lie. In the final stage it's possible to find the cake, but it's got no clipping so I think it's a hologram. The cake is a lie!
Oh, and Kirby, I was bored a few weeks back and ran that song through Google Translate and used the audio button to make the Google Translate voice sing the song . . . Not very well. >.<