Microsoft announced this Tuesday that the next major version of its Windows Phone operating system, code-named Mango, has been delivered to handset partners so they can get ready for a fall launch. The update brings a number of much awaited new features, including an HTML5 version of Internet Explorer 9 for mobiles, deep Twitter integration, better multitasking support, an improved messaging system, new Bing features and more.

"This marks the point in the development process where we hand code to our handset and mobile operator partners to optimize Mango for their specific phone and network configurations. Here on the Windows Phone team, we now turn to preparing for the update process," said Terry Myerson, Microsoft's corporate VP of engineering for Windows Phone.

Microsoft unveiled Mango in May, promising it will be available by year's end for free to existing Windows Phone 7 customers, and will ship on new phones from Samsung, LG, HTC and a few other partners. Nokia, which agreed to switch to Windows Phone as its primary platform, will also offer multiple Windows phones; the first one is codenamed Sea Ray.

Although it's no secret that Windows Phone 7 hasn't been selling well, many believe things will start to pick up with Mango and as the Windows Phone Marketplace gains more apps. There's no set date for a launch yet but today Microsoft Japan, mobile carrier KDDI and cellphone maker Fujitsu Toshiba Mobile Communications announced what will be the world's first Mango handset: the uninspiringly named IS12T.

The device, which will also be Japan's first to carry the Windows Phone operating system, will feature a 13.2 megapixel camera with autofocus, 32GB of flash storage and will be launched in September at the earliest. It will be a waterproof handset available in three different colors and will be powered by a Qualcomm Snapdragon MSM8655 processor.