Details are being kept to the bare minumum – all we have is speculation.
The folks at Microsoft sent out an invitation to the press yesterday to invite them to a media event on Monday, June 18th. The company has said nothing about what the event will be, aside from "This will be a major Microsoft announcement — you will not want to miss it."
Mashable writer Lance Ulanoff suspects that we'll see something right out of left field, like a Microsoft branded TV. There's also a possibility of something gaming related, but with E3 just a week behind us, it would be surprising. Perhaps this is when we'll see the reveal of the new console.
We don't know what it is but we can guess what it's not. It's not Windows 8, because that's not much of a surprise. It's probably not Windows 8 for phones, unless Microsoft is trying to do an Apple style conference. And it's definitely not a new Zune.
We'll find out in a few short days, but we're hoping for something great!