It always seemed strange that the Windows Phone logo was a shiny sphere considering that the entire Metro UI aesthetic is built around minimalist square/rectangular tiles, and it seems that Microsoft understood that. Reports are in from those out there lucky enough to be running the beta builds of Mango that the Windows Phone logo has become much more Metro UI.
As you can see in the images below, on the left is the old Windows Phone logo, and the right are the new square designs. It seems a lot like the recent change in the Chrome browser icon, moving from a shiny 3D sphere to a flat, minimalist logo. And, it is a good move on both accounts. Google is known for minimalist UI, and Windows Phone Metro UI is built on minimalist tiles. In both cases, the logos moved towards styles that make more sense for the products.