Check here for more info: http://www.enworld.org/forum/showthread.php?534241-WOTC-teasing-planescape-com
The Wizards D&D Twitter account tweeted this: "A countdown timer was recently brought to our attention. Plan Escape? Anyone know what this is about? http://planescape.com/"
Timer runs out on the 28th, so not an April Fool's thing.
Secret message on the page's HTML source code has hexidecimal code for ASCI characters spelling out: PSTEE (Planescape Torment Enhanced Edition?).
Website is hosted on same server as https://www.baldursgate.com/