Windows Mobile 7. Here's what he's saying via Twitter: the OS has been built on scratch, and none of the old WinMo 6.x apps worked. There's "a lot of horizontal movements, a lot of additional info by clicks... for WM users it will be a great step ahead. For market it's a copycat of Android 3.1/3.2 [sic] or iPhone." No direct comparison to Zune HD, except to say that it's a simpler UI than what he was seeing here, and also no clue on the kernel underneath. Still, the lack of legacy compatibility is either a huge bummer to the fan base, or if not that then Microsoft is preparing for a two-platform WinMo world. Then again, this is a purported early build (and while we're at it, an unconfirmed hands-on), so who knows what's bound to change before the final release. There's a lot of other details if you're willing to check his feed, or just head over to WMPoweruser for the abridged / "good parts" version. Boy, February just can't come soon enough.