After weeks of silence, Microsoft has finally confirmed that it will announce the first AT&T-based Windows Phone 7 devices on October 11 at a previously schedule Open House event. The devices will formally launch about a month later, on November 8. This information was previously and exclusively revealed in my Windows Phone Secrets blog.

"Join Steve Ballmer, CEO of Microsoft, and Ralph de la Vega, President and CEO of AT&T Mobility & Consumer Markets, to hear the latest about Windows Phone 7," an invitation to the event reads. After a morning press conference, the companies will provide a product showcase featuring Windows Phone 7, Zune, Bing, Windows 7, Xbox 360, Kinect, and more.   

As I previously reported, Microsoft holds this Open House event annually in New York City as a way to familiarize the mainstream, non-technical press with its consumer-oriented products. So when The Wall Street Journal reported last week that Microsoft and AT&T would "launch" Windows Phone on the same day as the Open House, and in the same city, I checked with my sources. Not so, I was told: Windows Phone 7 is launching in the US in early November. The October 11 event is a "press break," or an announcement, at which AT&T will reveal its initial Windows Phone 7 offerings. The announced devices won't ship until a month later.

AT&T is expected to reveal three Windows Phone 7 devices at the event, made by HTC, LG, and Samsung. The Wall Street Journal has reported that AT&T will be the exclusive US launch partner for Windows Phone in the US. If true, other carriers will presumably be added over time, given Microsoft's stated goal of having the system ship via all major wireless carriers.

Microsoft first announced Windows Phone 7 as the successor to Windows Mobile in February at Mobile World Congress in Barcelona. It then proceeded to deliver several iterations of the software and associated developer tools, finalizing the code for the new mobile OS in early September. All along, the software giant has been particularly cagey about the release date for this new system, noting only that its partners would deliver the first devices to consumers by the end of the year.

I'll be attending the Windows Phone 7 announcement next week and will report from the event.