Enter Zoneminder, which can use complex zoning trigger algorithms. But zm is a horribly complex thing, using Apache web server, Perl scripts, a database, and who knows what else. I spent hours on it, following dozens of recipes. In the end I gave up.
It's somewhat ironic that I was doing this on my old media PC. I got the Zotac years ago, when Sony went all nutso with the PS3. I ran xbmc on it, and it served movies to the big screen. It was also horribly complex since it had to deal with endless variations of hardware and Linux distributions. The update process was unworkable, and finally I gave up. It was all replaced by a tiny WD box from Costco.
It's fun being a pioneer, but eventually all this gets embedded in a special-purpose box, using Linux. Soon Zoneminder will be incorporated into a cheap Chinese camera that can connect to your network. Oh well, on to the next thing....
Update: Whoopeedo. I got 'motion' working. It set up real nice with a standard install. The hard part was making the old media computer a headless, wireless server.
This is fixed up a bit by Youtube.
It show that it records only with major movement.
Update2: That camera is useless at night because of internal reflections. I've looked around and have yet to find my perfect outdoor wireless ip camera.
Update3: I'm waiting until Spring to install my ideal camera - dome, ip wireless, 1080p, night vision, etc. All for very cheap. :)