Microsoft has sent around invitations for an event to be held in San Francisco on the 30th, and the invitations say "Join us to hear what's next for Windows and the enterprise." The event will be led by Microsoft Corporate Vice President, Operating Systems Group at Microsoft, Joe Belfiore, and Executive Vice President of Operating Systems, Terry Myerson. Microsoft is expected to show off and release a "Technical Preview" of the next major version of Windows.
We don't know a lot about Windows 9. We know that it is expected to unify Windows from phone to desktops, and may change the UI focus for different devices. Desktops will get less of the Metro UI, and more of the traditional desktop, complete with a redesigned Start menu, and tablets are expected to see less of the traditional desktop and more of the Metro UI. There has even been a new rumor that Windows Phone could drop the word "phone" from the name.
source: The Verge