A new login screen has been discovered in both builds 9888 and 9901 which were leaked in December, and you can enable it with a simple registry tweak - it isn't without bugs, however.
Insider preview boosts ransomware defence, fixes memory leaks, more
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Build 9901 RSS
Official info has come down from Microsoft regarding yesterday's leaked Windows 10 build. Apparently those who install it are breaking their automatic updates for future builds in the Insider Program.