Craig:
I've got two good sized dogs, a German
Shephard and a St. Bernard (why do I even
need an alarm?) Anyway, I managed to
overcome the motion detector problem by
installing the motion detector upside down.
The beam comes out roughly straight out
from the top of the detector and at a 45
degree angle or so out from the bottom, so
if you flip it and mount it about half way
up the wall, anything beneath the motion
detector won't set it off. You may have to
play with angling it and whatnot, but I've
had my two detectors this way for a couple
years and the dogs have never made a false
alarm that I know of.
This won't protect you from midget burglars
or crawling ninja-thieves ... but it's a
good start.