Musical accompaniment for this post)
The Portal 2 ARG is coming to an end, but GLaDOS, much like Senor Chang, can never die. Also, she has something planned with a final countdown.
The Aperture Science page seems to imply that once the "final test" is solved Portal 2 will be unlocked early on Steam: "Tomorrow you will be given the final test. Then it will be entirely in your hands when I am freed."
Despite feeble math skills and general ineptitude with time, we've calculated that the countdown ends at noon Eastern (9AM Pacific, 5PM UK). Our advice? FAIL THE TEST, because some of us have to work today.
Update: There are more cryptic details being posted on the Valve forums that lead to the idea of an early PC release. "TWO HOURS TO NEXT TUESDAY (UNLESS OTHERWISE REDETERMINED)." Add that little log to the ARG fires, you monsters.