I believe that it was Apple (and leaks/news of this possible hybrid) that tempted Microsoft to rush the Surface Pro 3 only 8 months after the SP2 was released. If Apple announces a hybrid/iPad Pro on Tuesday, I'll be more convinced.
They did. It was called Windows 98 tablet PC edition.
It was reported the iPad in its first few months of sales outsold all the Microsoft Tablet "editions" of the previous 10 years from all the manufactures combined - Compaq, HP, etc..
They would simply create a software based ARM emulator and switch between this and the underlying x86 physical architecture. Technically, that would be fairly straightforward.