Something on Steam on Dec 20th
 

Portal 1 Had a 50Mb update very recently, and this file was found within that update.

Speculation dictates that it's when the Steam Holiday sale will start. Also, follow the note of Killing Floor, Holiday themed maps might be available for the Challenges on Portal 1 and 2.

So, if you hear anything about other valve games (portal 2, TF2, HL series etc) post it here... maybe it's just a small hint for the sale, maybe something extra?

Just 4 more days until everybody gets a little bit poorer!

It depends...

If other valve games get ARG stuff, that may be hints towards a sale. But if it's only Portal that gets this, it may be a hint towards something Portal-related in particular...

Well, the tip that accurately pinned the autumn steam sale also pegged the holiday sale for the 20th, so I won't be surprised if the sale starts then.

There will likely be both a December 20th sale and something Portal related. I'm expecting that something is DLC as it's far too soon for Portal 3 announcements.

Could be an end-of-the-world joke, considering the date... Just before the prediction's supposed to be fulfilled.

Reading around on the Portal forums, it seems that image actually came from the back of a British magazine called GamesTM, and is in the part where they usually announce the date of the next issue (there's no mention of finding it in the game files, the recent 50mb update was to add Big Picture support). It does seem odd that they'd use a Portal joke now for seemingly no reason, though.

I'd love it if 21st is full of end of the world set games going on sale with some tagline like "to help you prepare"

Kind of sick of portal stuff when all I want is half life.

Maybe they're trying to advance the portal storyline enough to be able to tie it into half life. Or at least that's the only reason that we would still get a new half-life after all this time.

Half Life 2 was Valve's last attemp at creating a serious continuous brand history for a Single Player game. Portal was the Happiest of accidents and it only connects to itself... so, i no longer hope for HL3, first cause there isn't enough money in it for Valve to justify making it a really good game compare to other's in it's class.... and secondly cause it would probably suck.