Next year there will be 6 companies making mobile devices using Windows RT.
Microsoft, Nvidia, Qualcomm, Texas Instruments (though they may not get many designs due to exiting sector), Samsung and STMicroelectronics.
While it is highly probable, and most likely, that MSFT will engage in producing mobile devices, ala Apple, it will still need these other companies for Windows 8 and future generations for market penetration.to ably compete with iOS & Android.
Microsoft announced yesterday this is a "new era" for them as a "device and software company". Just look at the mere existence of Surface tablet as proof of that. Next will be the Surface phone.
Apple has proven it has the dominant model for mobile and MSFT is copying it. Google will be at a disadvantage at that point. MS's strategy will clone Apple's including making their own ARM chip for which they have already obtained an ARM architectural license like Apple to do so. The cost and complication of a fragmented WinRT system that has to be tailored to multiple vendors will disappear.
Relying on Asus, Acer, Dell, HP, HTC, Nokia, and Samsung to do battle against Apple is a failing strategy. MSFT is employing the winning Apple model from here on out.