Microsoft today released Windows 10 Mobile Build 10581 to Insiders. However, the company is also planning to release a new Windows 10 desktop build to Insiders on the fast ring today. The upcoming build will be Build 10576, and Microsoft will share more details this afternoon:
@GabeAul So now for the "thing we've never done before" that I tweeted about yesterday: New PC build will ALSO be out today. This afternoon!
— Gabriel Aul (@GabeAul) October 29, 2015