There are 26 radios spread through Portal. When you take them to a specific location within the area/puzzle you find them, morse code/other stuff comes out. Do this with all 26 to get an achievement.
Some of the transmissions can be turned into images (it's SSTV stuff). Decoding everything leads to a BBS number. Connect to the BBS via old school modem stuff to find a GLaDOS version 3.11 log in screen.
Using u/p backup/backup, ASCII images and weird messsages started feeding back after a while.
People are still getting new stuff when they log-in apparently.
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Among the theories on what's being revealed:
- Stuff from Ep3
- Stuff from Portal 2
- Portal 2 and Ep3 will be same game
- Announcing Gabe getting an award during GDC
- Announcing Valve presentation during GDC
- Announcement regarding Portal 2 or Ep3 or both during one of the above GDC things
It's been hilarious to follow the threads on this on the Steam forums.