Arne Hess, from PPCW.Net says that he knows the identity of the new Microsoft partner using Windows Mobile platform. The sources and rumours say that it's a company with wireless experience, but not necessarily mobile experience.
The folks at PPCW.Net believe Sierra Wirelles is the natural name. They have already experience in the wireless area, and some experience with mobile devices.
Rumours are going wild on this topic. But it'll be just another day and we'll see the real name.