Microsoft has started teasing a “major announcement,” which is scheduled for Monday, June 18. According to the invite email, “this will be a major Microsoft announcement – you will not want to miss it.” The media event will take place in Los Angeles, California at 3:30 p.m.

The topic of the event is under wraps, which has led to much speculation. Will we finally learn more details about Xbox Music, which was first revealed at E3 earlier this month? The Windows Phone Summit is just two days after this event, so Microsoft might be trying to make the announcement ahead of the big Windows Phone 8 reveal. Then again, it could be related the Windows Phone 8 OS itself–which would be odd, considering the timing of the summit–or Windows 8 tablets. The latter is more likely, in this case.

Mary-Jo Foley doesn’t think it will be Xbox-related, as the invite came from Microsoft’s PR agency for Windows and Windows Phone. AllThingsD, meanwhile, seems convinced that it will revolve around Windows RT, the ARM-based version of Windows 8. The thing is, a few Windows RT tablets were already shown off at Computex. This could, however, be a great opportunity to go hands-on with Windows RT tablets, as well as finally learn additional details about the OS like the built-in Office suite.

The location and timing of the event is certainly strange, and the words “major announcement” obviously implies that whatever is revealed will be of great importance. What in the world could it be?

[ZDNet]