I am just learning how to program a PIC myself, but what I would do is put a voltage divider on a logic input. The input will be high until the battery drops to the point that the divider output is at the switching point. The input will be noisy at that point, so depending on what you want to do, you may need to lock the input out once it has gone low.
like russlk suggested using a voltage divider , and o/p of that given to the external INT pin , which has schmitt trigger i/p, or connecting them to internal comparators.
The 16F628A has two internal comparators and a programmable voltage reference, so you have all the parts needed to make a very accurate low voltage detector, read the data sheet.
I've not looked at the circuit, but 'very high resistance' (NOT impedance!) is only of value if the total current consumption is already extremely low. There's not a lot of point saving 5uA if the project already takes 100mA.
As already suggested, use a PIC comparator - available in many of the newer PIC's.
Hmmmmm. I'm not so sure about this - I think you can use the term "impedance" when discussing DC, although it is exactly equal to resistance.
That's not to suggest that the two terms are interchangeable, obviously.