> Use a diode off one side of the mains transformer's secondary, through a
> largeish resistor, to a PIC pin, to detect AC cycles.  (Keep the current
> through the protection diodes low so you don't mess yourself up, by
> large enough resistor value.  100kish or so?)
>

uh, guys...
    There are some impressive voltage spikes running about on
the power lines. An optocoupler is a better solution, and you
presumably will need some isolation anyway at the output end
if it's to interact with a human.