The Surface is like a Nexus Tablet, but for Windows 8. Microsoft’s own attempt to create an incredible, competitive, tablet.
For certain, it is a risky strategy. As shown by the Surface, they evidently feel that they need to kickstart the cavalcade of Metro devices with their own, Microsoft branded, device. The product they have made is undoubtedly interesting (in particular, the ‘Smart Cover’ accessory that has an inbuilt keyboard) and probably captivating to a reasonable number of consumers.
However, whether it is wise to, essentially, become a competitor to their other “partners” (such as HP, Dell and Asus) is still to be determined. I think Google can get it away with it with the Nexus series because Android has already established platform lock-in, such as the app ecosystem. This makes it hard for a manufacturer to disconnect themselves from Android, despite their annoyances regarding the special treatment Google has given to some companies, and go it alone with another new OS.
For Windows 8, though, there is none of this. This is a platform with zero apps and zero install base. Potentially turning away hardware partners, before the OS even ships, is very risky, indeed.