To answer a phone call it's not really necessary to "slide": if you touch "all left" or "all right" the slider you get the desired action. I think that the correct behaviour would be to "grab" the slider with the finger and actually drag to the sides of the slider.
With slide2answer installed, since there is very little risk of accidental answers, the screen could be immediately unblanked regardeless of the state of the proximity sensor. Is this a feature that could be implemented in your piece of software?