The cake is a lie. The end credits song from Portal, Valve's gem of a game, is a less-than-subtle hint that villain GLaDOS, whom you supposedly just defeated, actually survived your encounter.
"Still Alive" is hilarious yet sinister; so basically a perfect reflection of its singer, GLaDOS. The great contrast between simple, happy music and the darker meanings behind the lyrics is a highlight of video game writing and creativity in general.
Whilst she might appear to be fairly harmless if you're just casually listening to the song, GLaDOS is excited to use all the new things she has learned dealing with the player on all the other subjects who are "still alive".
GLaDOS's passive-aggressive remarks towards the player's success make the song even more entertaining - she's "so happy for you", despite the fact that you tore her apart and burned her in a fire.
There could hardly be a better way to end Portal than with "Still Alive". It keeps the game's wonderful humor alive right to the end, effortlessly sets up a sequel and simply cements itself as one of the best original video game songs to ever exist.