Why does a sweaty finger neutralise the touch-sensor on my phone screen?
Most phone screens are capacitive. They work by sensing the change in capacitance due to a nearby conductive object.

I can't imagine a way in which simply having some sweat on a fingertip would interfere with this mechanism. I can't reproduce the problem with my phone and a glass of salty water.

However, I'll offer a guess as to what's happening: if you get enough sweat on the screen, the phone will have difficulty distinguishing what is a finger, and what is a gob of sweat. To the phone, broad contact over large areas of the phone look a lot like the side of your face, so the phone ignores these touches since mashing keys with your face is usually not desirable.