How can I interface to a capacitive sensor and measure its value with my Netduino?
you have several ways to measure the capacity :
1- make a oscillator and measure the period or the frequency  (use a CMOS Inverter with resistor connected form output to input and connect the sensor between input and GND) or use LM555 in astable circuit. The period of output depend directly on the value of sensor capacity (T = K * C) where K = Constant .
2- If you apply a step function on R-C network and measure the time t until the C voltage to a reference (Vref) you can use the equation : Vref = Vin * (1 - Exp(-t/Taw)) ; [Taw = R*C]. then t/Taw = -Ln(1 - Vref / Vin) ==> C = -t / [R * Ln(1 - Vref / Vin)] 
3- if you use a constant current source to charge the capacitor, the C voltage become : Vref * C = I * t ==> C = I * t / Vref . You can use a LM317 to make a current source,  a MCU internal analog compactor to compare the sensor voltage to Vref, a MCU internal voltage reference or TL431 as external voltage reference.