According to an article on theverge.com, this recently announced tablet will be running Windows 8.
Here is a small portion of the article and the rest can be read by following the link below.
Microsoft's Intel-based Surface tablet will run Windows 8 Pro, with a thickness of 13.5mm, a weight of 1.9lbs, and USB 3.0 support. This particular version will also include magnesium casing and a built-in kickstand, but will ship with either 64GB or 128GB storage. Additionally, the Intel version will include additional digital ink support through a pen that magnetizes to the body of the tablet, and a 10.6-inch ClearType "Full HD" display. Both of Microsoft's Surface tablets feature optional Touch and Type keyboard covers.
10.6-inch Microsoft Surface tablets announced, powered by Windows 8 | The Verge