The official Bluetooth Special Interest Group site shows a listing from Microsoft for a previously unannounced "Surface Ergonomic Keyboard," suggesting that the company plans to expand the Surface branding to more PC accessories.
The Bluetooth SIG listing (via WinFuture.de) doesn't include any more info. However, if such a product is indeed in the works, it adds more credence to the rumors that Microsoft is working on a Surface-branded all-in-one Windows 10 PC as well. Such a product would certainly benefit from a Bluetooth-supported keyboard.
Microsoft is rumored to be holding a hardware press event in late October, and if true we could see this new Surface keyboard revealed at that time. Microsoft has already been selling wireless ergonomic PC keyboards under the "Sculpt" brand.