We've been able to test out the basic version of Windows 8.1 since late June. Now there's a preview version of the enterprise edition, which include support for Windows To Go, AppLocker and other business-centric features.
Several of those features are additional to the enterprise features Microsoft announced at TechEd North America. There's no official word on when the final code will ship, but OEM code for the main 8.1 release has been promised to hardware manufacturers by the end of August, which points to a September-October release.
While Microsoft is moving to a faster rollout cycle, it isn't changing its main support parameters. Windows 8.1 runs under the same lifecycle parameter as Windows 8, with extended support for both ending on 10 January 2023.