The Cake

Motivation. How would running through test areas with a portal gun benefit the guinea pig? You are promised that if you accomplish your task, that there will be brief counseling, followed by a party and that there will be cake at said party. However, there are several theories about the cake.
1) It's a lie, as written several times on one of the walls (This is where the common internet phrase "The cake is a lie" comes from).
2) There is a cake shown at the end of the game, but whether it is THE cake is in question.
3) That YOU are the cake; at one point in the game, GLaDOS says "You will be baked, and then there will be cake," supporting this theory. However, she could also be referring to the point where the test subject (you) are to be dropped in a pit of fire at the end of the 'experiment'.
4) That it is a prank, because the contents of the cake are, according to her information core;
One 18.25 ounce package chocolate cake mix, 1 can prepared coconut pecan frosting, 3/4 cup vegetable oil, 4 large eggs, 1 cup semi-sweet chocolate chips, 3/4 cup butter or margarine, 1 2/3 cup granulated sugar, 2 cups all-purpose flour. Don't forget garnishes such as: Fish shaped crackers, Fish shaped candies, Fish shaped solid waste, Fish shaped dirt, Fish shaped ethyl benzene, Pull and peel licorice, Fish shaped volatile organic compounds and sediment shaped sediment. Candy coated peanut butter pieces, shaped like fish. 1 cup lemon juice. Alpha resins. Unsaturated polyester resin. Fiberglass surface resins and volatile milk impoundments. 9 large egg yolks. 12 medium geosynthetic membranes. 1 cup granulated sugar. An entry called 'how to kill someone with your bare hands'. 2 cups rhubarb, sliced. 2/3 cups granulated rhubarb. 1 tbsp all-purpose rhubarb. 3 tbsp rhubarb, on fire. 1 large rhubarb. 1 cross borehole elector-magnetic imaging rhubarb. 2 tbsp rhubarb juice. Adjustable aluminum head positoner. Slaughter electric needle injector. Cordless electric needle injector. Injector needle driver. Injector needle gun. Cranial caps. And it contains proven preservatives, deep penetration agents, and gas and odor control chemicals; that will deodorize and preserve putrid tissue.
Of course, this could also be due to the fact that GLaDOS is slowly losing her mind piece by piece.
5) That it is a metaphor for an unobtainable freedom from GLaDOS' experiment.
The truth behind the cake may never be known.