There are two things that can kill an inside joke:
1. A better joke: When portal 2 comes around or any other game with such succes and emphasis on humor people will switch to new jokes; it might get some revivals when there is a perfect setup to employ "the cace is a lie" as situational comic
2.Time...In a few years there is another gaming system, everything looks even better and the mayority of gamers has never heard of portal so they will ask "why is the cake a lie?" and every joke turns bad when you have to explain it...
But to use this opportunety: Which are the other succesful memes gaming has inflicted upon us? I started playing many years ago but I have missed out on any jokes during the dos era and before...
The ones I can recall at this moment:
-There is no Cow level
-Chuck Noris ... (this one has gotten out of control)
-...when its done (duke nukem developemend team)