The motion software is a proven solution -- it's pretty much the defacto motion detection software for Linux. I've used it before to monitor the cat food dish, to find out whether my cat was eating after having been sick. Setting it up was pretty easy, and it does have a webserver built in. If you're going to roll your own solution, the most difficult part is going to be the hardware side of things -- finding the weatherproof cameras, and getting them connected up for motion to read. A quick google turns up this, which, though not a webcam, could hook into a video card.