As expected, Gabe Aul just pressed the big red button this morning and released Windows 10 Mobile build 10581 to the Windows Insider fast ring.
— Gabriel Aul (@GabeAul) October 29, 2015
You can currently upgrade directly to 10581 from not just Windows Phone 8.1, but also on phones running either Builds 10549 and 10572. It is a much appreciated change that you don't have to flash your phone back to 8.1 to be able to upgrade to this latest build.
Gabe stated that while this build doesn't have any significant new features, there are a number of good fixes and general improvements. Stay tuned for more info as we get a hands on with the latest build and report the fixes and known issues with 10581.