The Potato Fools Day ARG resulted in an early unlock for Portal 2, but the clever tricksters at Valve have shown how far ahead they planned by hiding ARG notes inside the game itself. The ARG Wiki notes a hidden commentary node in Chapter 3, voiced by Adam Foster. Inside the sound file are three images that outline the ARG that just closed with the game's release.
It doesn't do much good to the ARG now that it's closed, but it's a nice little wink at fans to assure us that this wasn't made up on-the-fly. Another well-hidden easter eggs includes an image of the Companion Cube embedded in the sound file that appears upon completion of the achievement "Final Transmission."
And though it isn't technically a hidden easter egg in the game, per se, PC players were surprised to find an interactive trailer for the upcoming film Super 8 in the Extras menu yesterday. The short stage was built in Source, and lets the player explore the train and surrounding area before, during, and after the crash that drives the film's plot.
If you've spotted any other hidden goodies in Portal 2, give us a shout in the comments to let your fellow Shackers know.