UPDATE: Valve Alters Portal
UPDATE 2: Mystery solved! Well, sort of. Not all of the decoded info is entirely clear, but we do know one thing: Portal has a new ending. After a second update to the Steam version of Portal released this afternoon, Xfire user Exilion posted this video, which showcases the brief addendum to the traditional ending of the game.
It is not necessary to get the new "Transmission Received" achievement to see the new sequence. Instead of the screen simply fading out after GLaDOS' apparent destruction, the main character (Chell) is dragged away by an unseen Aperture Science robot – presumably the "party associate" mentioned earlier in the game. The robotic voice says: "Thank you for assuming the party escort submission position."
It seems pretty clear that Valve isn't adding to a 2 1/2-year-old game just for the fun of it. The smart money is on an announcement from the studio sometime in the near future.
UPDATE 1: The mystery deepens! What started as one new achievement in the Steam version of Portal has unfolded into a whole multimedia puzzle. Images, sound clips, and various numbers all seem to be hinting at strange happenings in the Portal universe. Also, a correction to the original story: It turns out that while there are new radios hidden throughout the test chambers, they don't all involve standing on buttons. Instead, players need to bring the radio to a variety of specific locations throughout the game to trigger the new transmissions. The Morse code (and other data, like the image below) that unlocks has been collected by users in this rather cluttered location. It's definitely an exciting time to be a Portal fan.
One of many cryptic new images
Users at Valve's official Steam forums have uncovered a stealth update to Portal, adding a new achievement and some suspicious new audio to the game.
The new achievement is cryptically called "Transmission Received," and starts tracking your progress at 0/26. In this thread, Steam users have deduced that players need to tackle a new task as they play through the game: pick up radios and stand on buttons. New radios (with red lights) are hidden in the test chambers. By picking them up and standing on the red buttons found throughout the trials, the radio light turns green, triggering segments of Morse code just begging to be deciphered. Presumably, that will involve holding a radio and standing on 26 buttons.
Additionally, user PositiveJosh posted this sound file, found by diving into the game's assets. It features a male robotic voice "The enrichment center thanks you for assuming the party escort submission position."
What does it all mean? Speculate in the comments below!