Today’s the big day for all those Windows Mobile fans in the U.S. Microsoft today announced official availability of three Windows Phone 7 devices on two different carriers.

According to Microsoft’s online storefront, the HTC HD7 (T-Mobile USA), HTC Surround (AT&T) and Samsung Focus (AT&T) are all available today. All three phones sell for $199 with a two-year contract at their respective carriers.

The Samsung Focus on AT&T features a 4-inch Super AMOLED touchscreen, 1 GHz processor, 5-megapixel camera with 720p video capture and expandable memory up to 16GB. The HTC Surround at AT&T features similar specs to those of the Focus but offers a slide-out qwerty and 3.8-inch touchscreen. Over at T-Mobile, HTC HD7 is a near clone of the Surround being offered at AT&T.

Microsoft has been climbing a steep hill over the past couple years as the company struggled to get its mobile OS up to speed with competitive offerings like Apple’s iOS and Google’s Android.

In research from September 2010, Gartner projected that Symbian and Android will account for 59.8 percent of mobile OS sales by 2014. In the same report, Gartner forecasted that Windows Phone will account for just 5.2 percent of the mobile market by 2011 but decrease to 3.9 percent of the mobile market by 2014.