Portal 2 ARG event now completed, some of the Potato Sack indie game developers involved in the unique marketing event for Valve's puzzle-shooter are taking about their involvement. Two Tribes, who had two games in the the ARG (Toki Tori and RUSH) posted up an article on their web site that talks about putting the Portal 2 themed content into their games over the length of the ARG.
Othe developers like Gaijin Games (Bit.Trip Beat) and Hidden Path Entertainment (Defense Grid The Awakening) also talk about their work on the ARG which started out with Valve inviting all the developers last December to work on the project. The result was a ton of extra free content for all 13 games (levels, puzzles, and more). Much of it will remain in the game after the ARG is finished.