Now that the GDR3 update allows Windows Phone manufacturers to build phones with the same exact specs as any high-end Android model, it is possible to conceive of an OEM using the exact same handset (with some minor tweaks) to offer both an Android and Windows Phone top-shelf model with the same specs.
Virtual buttons have been leaked for a Windows Phone model before. Before the launch of the Nokia Lumia 800, the very first Windows Phone device was code named the Nokia Sea Ray, and had virtual buttons before a design change replaced them with physical buttons.
Leak reveals support for virtual navigation buttons for Windows Phone Blue
1. Leak shows layout of virtual buttons supported by Windows Phone Blue
2. Nokia Sea Ray, later the Nokia Lumia 800, had virtual buttons
source: @evleaks via WMPoweruser