A new rumor suggests that Microsoft is planning to show off a Windows 8 tablet UI by the end of the company's fiscal year. Microsoft's fiscal year runs from July 1 to June 30, meaning that we may see the demos in question sometime in the next three months. The tidbit comes from "a source at Microsoft" quoted by Business Insider:
This person claims that Microsoft is taking a more Apple-like approach to interface design this time around, and will also be using concepts from the "Metro" interface developed for Windows Phone 7. Ironically, that interface grew out of the Media Center UI that shipped with Windows XP back in 2004, but the Media Center interface was meant to be viewed from up to 10 feet away and controlled with a remote control, whereas Metro is meant to be used with touch screens.

A rumor from just earlier this year suggested that Microsoft has already compiled Windows 8 Milestone 2. This would suggest that Microsoft might want to show off Windows 8 Milestone 3, which would be still very early in the development cycle.

Another rumor also from two months ago suggested that Windows 8 will include a new tile-based user interface, codenamed Mosh, as well as a new app model, codenamed Jupiter, that will target a new Windows Marketplace app store. Let's add more rumor tidbits from three months ago: the OS will have two user interfaces, a new fast hibernation system, and will intelligently manage an SSD.

Please remember these are all rumors. They won't all be correct, and even the ones that are will not be completely on the money.