Asus had previously created a PadFone that used Android at its core, so extending that concept to Microsoft's ecosystem isn't that much of a stretch. It's even possible that the device will have some sort of hybrid between Windows Phone 8 (when in phone form) and Windows RT (when in tablet form).
In addition to Windows Phones, Asus is trying to establish a stronger presence in the U.S. mobile market and is in talks with network providers to bring some of its existing hardware stateside. They might achieve just that with their innovative hybrid products.
via Tech Crunch