Pulling another page from Apple's playbook, Microsoft has sent out a cryptic invitation to lure reporters to an upcoming event, while offering almost no clue as to what will be introduced.

In this case, Microsoft has invited select reporters to an event on April 12 in San Francisco. Engadget appears to be first along the mail truck route from Redmond, Wash., so for now you will have to go there to see pictures of said invitation.
Microsoft is declining to comment, though I'd bet it has something to do with the mobile front. One possibility is the introduction of a Microsoft-designed consumer phone, which is on track to come out this year, ahead of the first Windows Phone 7 devices from other makers.
As interesting as what Microsoft has in store--OK, maybe not as interesting, but still noteworthy--is the evolution of Microsoft's consumer marketing to try to generate more buzz and a bit of a cool factor for its products.
For those keeping track of the lessons Microsoft appears to have learned from Cupertino, it's got better product naming, slick packaging, hip retail stores, and, now, mystery events.
Of course, Apple has its own mystery event in the coming days, where it is expected to introduce the next version of the iPhone OS.
Source : news.cnet.com
UPDATE :
Earlier today, Microsoft announced it will be holding a mystery event for their latest project. This mystery meeting was revealed today by Ina Fried from Cnet.
The invitations were sent out to members of the press with the message “It’s time to share.”
The mystery meeting is to reveal their “Pure” and “Turtle” phones, part of Microsoft’s Project Pink. The Pure and Turtle are mobile phones but do not run Windows Phone 7 that will be released later on in the year. The devices are a replacement for T-Mobile Sidekicks, manufactured by Danger, a company Microsoft acquired. The phones will support the Windows Mobile operating system, but not Windows Phone 7 OS, and come feature packed with social media outlets like Facebook and Twitter, including the Zune music service.
The Windows Phone 7 smartphones aren’t expected until the holiday season, an overhaul to Windows-based phones, featuring a Zune like interface.
The Pure and Turtle will both be slider phones, with touch screen and will go on sale within the month following the announcement on April 12. The devices will be offered through Verizon Wireless.