Microsoft has just sent out an invitation to an event and to use their terms, "This will be a major Microsoft announcement – you will not want to miss it.". While the details are slim at this point, Microsoft certainly has something major to announce on June 18th.
There is very little information to go on but a couple things to keep in mind. Microsoft is holding a Windows Phone developer conference later that week so it could be Windows Phone 8 related or it could be related to Windows 8, possibly the announcement of the RTM. We would be surprised if Microsoft would create this much noise to only announce the RTM of Windows 8, but anything is possible as we don't have much information to go on.
There is a lot of noise on Twitter about what the announcement could be but it is all speculation at this point. Just to toss around a couple of ideas (this is pure speculation) it could be Office for iPad, Windows 8 release date, strategic partnership with RIM, or even a free pony for all Windows Phone users. Your guess is as good as ours and we will bring your more information as soon as we get clarification from Microsoft.
AllthingsD has just penned up a post that claims it is for Microsoft to push Windows RT tablets. That's right, the conference will be all about Microsoft's tablets and their strategy with Windows RT. Is this correct? Well, it's the only thing we have to go on at this point, stay tuned.