The first step towards this event happened on April Fool's Day (or "Potato Fool's Day"), when a "Potato Sack" bundle of discounted indie games hit Steam. Over the course of the next week, secret messages began appearing in those games. Puzzle pieces were sent to websites (like ours!), which, when put together, revealed a picture of a bathysphere and a coded message suggesting thirteen "tests" would follow.
And they did, in the Potato Sack games. Portal 2 content was added to all 13 games on April 12, including themed levels in Amnesia: The Dark Descent, Bit. Trip Beat, and more. That content held even more secret messages, which led to "Aperture Science" login screens, and then to locations around Seattle -- which, plotted on a map, led to a code that turned out to be the name of a Steam group.
That Steam group held screenshots of Portal 2, each of which included a component of a QR code that led to a countdown site. That countdown elapsed, revealing "GLaDOS@Home," a parody of distributed computing efforts in which time spent playing the Potato Sack games contributed to an early release of Portal 2. And that's where we are now.
For a detailed history of the ARG, the Valve ARG Wiki has compiled a chronology of its investigation process. Although we're pretty sure you all stopped reading up at the top of the post, when we said Portal 2 had been unlocked.