Ahem. The Valve ARG wiki was the site that gathered and put together all of the clues that were sent out to various parts of the gaming industry/community during the ARG that led up to Portal 2's launch. Valve knew perfectly well a site would spring up for this purpose, and on a few occasions they actually meddled with the wiki themselves as part of the ARG. This afternoon, the following appeared on the wiki's news page:
"* 14:21: Something very fishy is going on in our filesystems, some countdown has appeared.. take a look here [http://valvearg.com/c/o/u/n/t/down/]."
Yes, this could still be a hoax. But since Valve explicitly said that the ARG still isn't over, I'm guessing it's not.
And no, I'm not affiliated with the wiki at all. It just irks me a little when people who didn't do the research tell other people to do their research.