For those who want to know more, this is basically the ARG that Valve has been doing coming to an end. Yesterday all the Potato Sack games updated with real Portal 2 content, as in Portal 2 themed levels and stages based around these particular games. We now know the 'story' of the ARG; GlaDOS hyjacked a bunch of independant developers and Valve, and is using their games as 'testing rooms' for 'us' to participate in. You can read a pretty good summary of events, from the perspective of it being a story,
here.
The biggest give away comes from
Gaijin Games, developer of the Bit.Trip series. Bit.Trip Beat is part of this ARG, and when their game updated with Portal 2 themed content they made a blog post. Of note is the bolded;