I agree with acosmichippo that we will only fully know when we try.
However, I feel that it won't be a problem with the manually put on screen protectors, but the built in protectors that are included with certain iPhone cases, those may possibly cause a problem. That's where they will hopefully create a updated case to accommodate for the added feature.
I know the camera flash is a different size on the 5S but the phone dimensions are the same so I think my speck candyshell should work for now....the front is open and the camera cut out is nice and big. Am I right?
Even with the nuud's method that's only the screen which is glass. This sensor like you said probably can't get wet and I doubt its made watertight around that ring anyway.
The only way is to make the case front seal around the sensor if there is no material transparent enough to cover but then you still have that exposed anyway. Also if the case front is more raised than the sensor it will create problems placing the fingerprint due to the distance.
Now if the sensor can still work through a thin membrane keeping it waterproof then I'm sure they can figure it out easily. But until LP gets a 5S to experiment with and that's gonna hold up R&D and production we won't see a 5S case for a long time
Not too worried about as I am over entombing my iDevice.
I do recall the scanner was "powerful" and that it reads the subdermal layers of your fingertip, so perhaps a thin membrane is not out of the question....
I have used an otter box in the past and it did leak. Later on I found out that it was a Chinese copy. I bought it through Amazon. I though I was safe but the copies look so like the original that to the untrained eye it is difficult to see the difference. They say, you buy cheap, you buy twice!