Microsoft is limiting the number of tablet manufacturers that can put Windows RT devices on sale. According to China Times, each Windows RT ARM chipset vendor can choose two OEM partners for the launch, with other potential tablet manufacturers having to wait until January for permission from Microsoft.
NVIDIA chose to include Lenovo and Asus in the launch, Texas Instruments chose Toshiba, and Qualcomm opted for Samsung. HP was also selected by Qualcomm, but they decided to drop themselves from selection in favor of working on the X86 architecture, and could be replaced by Dell.
The initial Windows RT tablets will be out on October 26th, at the same time as the official Windows 8 release. Aside from Microsoft's own plans with the Surface and Asus' intention to put the operating system onto its Transformer tablets, other manufacturers haven't officially confirmed their own Windows 8 RT devices.