It's from the game Portal, where you're put through a series of challenges by a vaguely homicidal computer that speaks with a fairly sweet voice. The computer occasionally encourages you to finish with promises of cake, though if you turn the right corners in the game, you find graffiti such as "The Cake Is A Lie!", presumably from previous players. Whether or not the cake is, in fact, a lie, you'll have to find out by finishing the game :)
The cube is the "weighted companion cube". You have to occasionally manipulate the weighted cubes, and in one level where you have to carry it for a long time, the cube has hearts on it. The computer warns you that some people have become unduly attached to their weighted companion cube, and advises you that it is not alive.
I only got to play it a little, but it's pretty fun. (I've got a roommate - Vance, who's occasionally posted here - who's a heavy gamer).
I have head of this game, but didn't know anything about it.