Valve has released a lengthy list of updates to various games today, lending more threads to the rich tapestry that is the Potato Fool's Day alternate reality game (ARG). Though unlisted in Valve's notes, Killing Floor has gotten an update with a Portal-themed level. Tripwire claims the level was "not yet due for release," but invites fans to play it anyway.
Other notables include updates for The Wonderful End of the World, 1.. 2... 3... Kick It, Cogs, Defense Grid, RUSH, The Ball, Bit.Trip Beat, AaaaaAAaaaAAAaaAAAAaAAAAA!!, Audiosurf, Amnesia, and Toki Tori.
The Valve ARG Wiki has already noted audio pieces in Bit.Trip Beat, The Wonderful End of the World, Audiosurf, and Killing Floor. The rest of the games are bound to hide clues as well. This is all part of an apparent marketing campaign for Portal 2, which kicked off on April Fool's Day when Valve released the Potato Sack sale bundle.
For Valve's part, they're not exactly playing coy with the connection anymore. The Valve ARG Wiki has also noted that a new banner directly connects Portal imagery to the Potato Sack bundle, and the company is offering a Portal 2 co-op skin for Potato Sack purchasers.
Unrelated to the potato sack game, Amazon tweeted a link to "follow the portals." Clicking through the various links will eventually lead to some Portal 2 videos, including two songs titled "Reconstructing Science" and "Robots FTW."