Check the specs for power consumption on those units. All I've seen are over 25+ mA. Other problem is they will detect ANY fairly solid moving object - not just 'warm-bodied' animals. Some might find that useful in specific circumstances.
A good use of the microwave detectors is to make a 'hybrid' system (I've got one on the drawing board). Use a regular passive detector AND a microwave detector. The passive is on 24/7. When it detects motion, it powers-up the microwave detector. Only when BOTH the passive AND microwave detectors have agreed that there is indeed something moving, will a pick be taken. This virtually eliminates falsing while maintaining low power consumption.
(Edited by Archilochus at 4:23 pm on June 4, 2002)
I've considered doing the same thing. I picked up a geophone that is pretty sensitive. Use the geophone to pick up footsteps and the PIR to pick up the moving heat. The geophone was pretty cheap, $2.50 and probably has $5.00 worth of scrap brass. Only fire the camera when there is a signal from both.