The "cake is a lie" is the key to understanding Portal's narrative meaning. Possibly referencing the nature of our world, where we are told from a young age that the world is our oyster and there is many splendors to be achieved. But in truth, the prize, goal and cake aren't actually achievable things, just the journey to that lie. However, is Portal referencing the journey to the lie is the prize.
In essence, is Portal trying to bring to mind the delusional world we live in, or bringing to our attention that the "cake" is a journey? (One take)
Another take on Portal's narrative: a warning of technological control.
All things are made by people, so why does Glados control? A likely conclusion might be he was made by the government to control, but he became too unstable and developed a mind of his own. His instability being a warning of too much technological dependence can turn on us. (Another Take)
By the way, me and kconley are taking a narrative approach to our study.