nicolai,
I'm sorry to bother you with this very specific question but here it is anyway.
My proximity sensor is no longer working.
I have tried replacing it with a new one but nothing changes (yes, I dissassembled my n900 completely and chaged it together with the front camera and all the stuff that comes together) so everytime I get a new phone call the screen of my phone remains blank and locked until I unlock it by hand.
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?
There is also a strange behaviour in your code that I would like to point out.
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.

Thanks anyway for this useful piece of software!