Playing through Portal, I like most other people found several of the hidden rooms where Doug Rattman wrote all sorts of things, including that the promised cake is a lie.
I realized that GLaDOS was just using the cake as false motivation. But then I beat the game and watched as the ending credits rolled, and in the very last scene, we see a cake, surrounded by personality cores and a companion cube!
Does this mean that the cake was not a lie?
As pointed out in the question,
the cake is shown at the end of the game.
During the final battle with GLaDOS,
a recipe for cake is given by one of the personality cores you destroy. According to the Portal Unofficial Wiki, the recipe contains "inedible or undesirable ingredients".
Finally according to the answer to this Arqade question:
When GLaDOS is defeated, she says the line "There really was a cake".
All this implies that the cake does actually exist. However it remains unclear whether GLaDOS ever really intended to reward Chell with cake for completing the trials.
While it's questionable whether it's canon or not, GLaDOS's song "You Wouldn't Know" from Lego Dimensions implies heavily in its lyrics that the cake in Portal really was a lie.
How? By repeatedly insisting that the cake she has now is real... as if she had lied about it in the past.
Warning: Spoilers for the plot of Lego Dimensions since it's the End Credits theme.
However, this isn't the only evidence. Portal 2 made everything after the Intubation Associate brings you back in the facility non-canonical. If you don't know what I'm talking about, this is part of the ending that was patched into the game in early 2011 in order to bridge Portal 1 with Portal 2.
Other than a few cores, the entire facility is shut down when you kill GLaDOS at the end of Portal 1. You bring GLaDOS and the facility back to life during the events of Portal 2.
What Rattman was trying to say was not that there isn't any cake; more that the (implied) idea behind the cake is a lie.
[Test Chambers 5-8] GLaDOS: As part of a required Enrichment Center protocol, the previous statement that we would not monitor the test area was a complete fabrication. We will stop enhancing the truth in three... two... zzzt
[Test Chambers 12-16] GLaDOS: Cake, and grief counseling, will be available at the conclusion of the test.
[Test Chamber 18] GLaDOS: At the end of the experiment, you will be baked, and then there will be cake.
So it's not so much about whether or not there will be cake, just that the cake isn't what you think it is.
