The cake is a lie

[snopes]

Comment: Last year the popular game "Portal" was released for PC and X-Box
360. One of the recurring themes in the game is that the promise of cake
is used to motivate the protagonist to complete tasks in each level. Late
in the game the protagonist sees writing on the walls in various parts of
the game saying "The Cake is a Lie!!" Since then this phrase has become
very popular among gamers.

Recently, I found this article on urbandictionary.com:

http://www.urbandictionary.com/defin...+cake+is+a+lie

One of the definitions seems to indicate that the origin of "the cake is a
lie" has its roots in child pornography. The suggestion is that people
would be "fooled" into opening a picture thinking that it was child
pornography only to find out that it contained some other image as a
"prank," similar to a "Rick Roll." Upon discovery of the prank, the users
would type the message "the cake is a lie" in the comments to warn other
users.

I was wondering if there is any truth to this. It's disturbing to me that
a phrase that has its roots in child pornography was popularized in a
mainstream video game. My instinct is to think that it's not true, but I
can't find any references either way (in support or denial) other than the
above article.