What is the meaning of the end scene in Portal?

In Portal, we are promised cake and a party at the end of our tests. However;

we soon discover that the cake is a lie.

Then, at the final scene;

there really is a cake (and my companion cube is waiting for me).

What's the meaning of this? Was GLADOS not deceiving me? Her comments seem to suggest that;

...she had the intention of killing me. Was the cake for us (player) and not for the character in the game?

Solution 1:

This type of scene is called a "stinger".

http://en.wikipedia.org/wiki/Post-credits_scene#Post-credit_scenes_in_video_games

In this case, the use of the stinger is to demonstrate that GLaDOS is definitely

Still Alive (since the cores are waking up).

The fact that a cake happens to be nearby is just a joke.

Solution 2:

GLaDOS is a psychopathic monster, and always intends to kill you. If you don't

escape the incinerator, or you fail to shut her down during the neurotoxin sequence, you die. She never has any intent to give you the cake or the party - these are just things she uses to entice you.

At the end, you can see that she could have made good on her promises, even though there is no outcome in the game where she does.

With regards to the cube, however (Portal 2 spoiler):

She really does have your cube, and she returns it to you in Portal 2 when she ejects you from the testing facility.