In a theatrical dimmer product (the Shoebox dimmer at
http://www.dovesystems.com), we just have a 10K resistor from 12VAC
(power supply transformer secondary) to the INT input of a 16c74a.  The
INT pin has a schmitt input, so there's some noise reduction there.  The
pin is programmed to generate an interrupt on a negative edge, so we get
an interrupt slightly before the negative going zero crossing.  We use a
timer to find the positive zero crossing.  This keeps the PIC isloated
from the power line.

Harold