Microsoft today took an unexpected $900 million charge to account for what it called "inventory adjustments" for the Surface RT, the poor-selling tablet that debuted last year. Its fourth-quarter fiscal numbers -- published on its website shortly after the U.S. financial markets closed -- pointed out the massive write-down.The company has been aggressively discounting the Surface RT, which runs the scaled-down Windows RT, a tablet-specific version of Windows 8 that relies exclusively on the Metro tile-based user interface and app ecosystem.