The Windows 8 and Windows Phone 8 share the same core, but in term of technology, they are 2 different products. One is for PC or tablet and one is for smartphone devices. We all know that Microsoft did show some Win 7 build that run on phone used ARM chip, but because at that point, the processor did not fully optimize for a big OS like Windows, so they needed to develop Win RT. Even if they actually develop full Windows 8 Pro on phone, what chip can utilize the power? At this point, there is only Intel chips but they are not really powerful enough to run the Pro version. If it is RT version, then it is possible. By few years later from today when mobile CPU - GPU can equal desktop CPU - GPU, they might combine Windows and WP to one product to run one multiple devices
My thought about the Surface is that the phone is just a backup plan if Nokia or HTC or any OEMs fail to get WP8 to consumers, but as 920 is selling like hotcake, it is another story. They might just look like Google that release a phone without any restriction from carrier and probably cut cost, but they don't have any telephone patents to backup, so either they need some OEMs to build for them and take half credit, or they purchase the OEMs and take all credit.