Microsoft has sent e-mail invitations to reporters far and wide about a mysterious announcement in Los Angeles on June 18.

(Yeah, that's Monday. And this is Thursday. Four days from now.)

"You are invited to an exclusive Microsoft media event in Los Angeles, California on Monday, June 18th. Doors open at 3:30 PM," reads the invitation from Microsoft's Waggener Edstrom PR agency.

I don't know what this might be.

One guess: Perhaps this is the Xbox Music unveiling. Xbox Music is the replacement for Microsoft's Zune Music service. Microsoft officials noted the importance of Xbox Music during the recent E3 conference when they shared an overview of theirSmartGlass Xbox companion app, but didn't discuss specifics about plans for ZunePass, multiplatform support or other Xbox music/video futures.

The reason I'm guessing this: It's in Los Angeles, so most likely entertainment related. And it's two days before Microsoft's day-long Windows Phone summit in San Francisco, where the Softies are expected to finally shed official light on what's in Windows Phone OS 8, a k a "Apollo."

But another guess: Maybe this is Microsoft advertising related. The Cannes Lions ad-fest is next week. Could this be some kind of a roll up of how Microsoft is helping partners bring new kinds of ads to even more of its properties? (Yay?)

But -- inside baseball alert -- Edelman is MIcrosoft's PR agency for Xbox. Waggener is Microsoft's agency for Windows and Windows Phone, among other accounts.This invitation came from Waggener.

Other guesses? (And don't say Yammer acquisition, unless you have a good reason why the pair would make that announcement in Los Angeles.)

Update, 12:41 p.m.: A couple of other sites (AllThingsD, for one) believe the announcement might be more of a glimpse of Windows RT tablets and services running on them. The timing kind of makes sense, as Windows RT, the operating system running on these tablets, will share a number of the core elements that are expected to be part of Windows Phone 8 -- which Microsoft is going to detail on June 20. Still, why Los Angeles, for a Windows RT media event, I wonder.... And why so late in the day (3:30 pm PT)...

This item first appeared on ZDNet's All About Microsoft blog under the headline "Microsoft to hold mysterious media event on June 18."