Hi,
If you go the ADC route I suggest putting a voltage divider in between Vdd and the ADC input. Otherwise you're overdriving the ADC input, the internal protection diode from the input to Vdd may be forward biased (depending on its actual Vf compared to the 1N4148 or whatever).

If the PIC has a comparator you may want to use that instead of the ADC because then you can set it up to trip an interrupt when the comparator output toggles - may be better than continously sampling the ADC. (The voltage divider still applies though.)

/Henrik.