It's not offering much in the way of hints, but Microsoft has just invited the press to an event in Los Angeles on Monday, June 18th for what it promises will be a "major Microsoft announcement." While it didn't have any more to say than that, the company's Windows Phone summit does follow later that week, and there's been no shortage of Microsoft-related rumors as of late, including a possible acquisition of Yammer (reported today by Bloomberg), and long-running talk of Office for iPad, which was again revived earlier this month. Whatever it is, we'll be there to bring you the news as it happens.
Update: Citing "sources outside the company," AllThingsD is reporting that the event will focus on Windows RT for ARM-based tablets, and that Microsoft will be showing off devices running the OS as well as some of the programs and services that will be available for them.
Microsoft Windows RT