They posted it on April 1, 2011. Given their history and level of credibility, however, I'm inclined to believe it when they say that it is not an April Fools' day prank, especially given that Microsoft has started pushing Windows 8 builds on Microsoft Connect.
The screenshot clearly shows a Metro-inspired logon screen. For those not in the loop, Metro refers to the design style used on Zune and Windows Phone. The screen shows the time, day of the week, the date, icons for power management, and an ease of access icon. The logon screen will be of course at least partially configurable, including being able to change the background image (this is probably pulled from the current Windows theme).
Microsoft will reportedly build on this logon screen, allowing for a "pattern unlock" for tablet users (you've probably seen this on Android devices), as well as audio controls for music so you can easily control music playback even while your tablet device is locked.
We'll be sure to keep an eye on what else Rivera and Thurrott reveal in the coming months. The Windows 8 leaks are coming, we promise.