The total length of the string is the length that determines the tension the string has to be brought to to attain a given pitch.
The bit behind the nut matters, as does the bit below the bridge if relevent on the guitar in quesion.
f=(1/2L).sqrt(T/d)
where f is frequency, L is the length of the string, T is its tension and d is the mass per unit length of the string, f is therefore inversely proportional to L (but we all knew that already, right?) and L has to be the total length. That means that if you increase the length behind the nut for the same pitch, tension as a funtion of length here is
T = 4df^2L^2
But we're keeping f constant d is also constant, and so was '4', last I checked, so may as well write
T~ L^2
= increase length, increase tension needed to attain the same pitch, so philly was right.
Sorry for the maths, but its a physics question, it gets a real physics answer :p.