I'm not sure if you've had any luck with this yet, but I had the same issue when trying to pair my Lumia 920 with my Surface Pro. Although it appears that they are not connected, the Internet Sharing does indeed work. Once you've successfully paired the two devices, ignore the fact that they disconnect a few seconds later. Next, click the WiFi icon in the system tray and you should see your Lumia's Internet Sharing network name appear (you may have to wait a few seconds for it to appear in the list). Simply click on it like you would any other network you normally connect to and you'll see that it works and your Lumia's WiFi connection is still off. However, like some else mentioned it does not automatically turn Internet Sharing off when you disconnect from the network on the Surface, but at least your WiFi radio remains off. There is now absolutely no need to have a separate data plan for your 8.1 tablet device if you have a Windows Phone 8 device with Update 3.