On the Full Disclosure mailing list comes this announcement from a group of anonymous security researchers:
During one week (2007-04-02/08), new undisclosed vulnerabilities / flaws / exploitation techniques discovered in the latest versions of the Microsoft Windows Vista operating system and softwares will be publicly disclosed on this page. This project is launched as a challenge by an unofficial team of security experts. Security advisories including advanced technical details will be provided. No more details will be provided before the beginning of TWOVB.
As with the Month of MySpace Bugs announcement, take with a grain of salt.
[UPDATE: April 3, 2007] That grain of salt would have been very useful. This was, as I suspected, a hoax aimed at the pen-testing the IT media. From what I've seen, the media came through just fine.