Passive IR motion sensors are triggered by body heat. I wouldn't be too quick to discount them. I don't think you'll get many false triggers from wind blown objects that are not emitting heat.
I removed the post because I had one of those "Doh" moments. Of course the PIR sensors work on a heat index.. Oops 
(I thought removing my initial post would cancel the thread).
No problem!
It's pretty easy to interface the Arduino to a PIR. Parallex makes a popular one (RadioSh** sells them) and here's another . . .
Interfacing it with the Arduino gives you lots of options.
Oh yes, I have a few already
Thanks!
This may be worth investigating...
http://www.zilog.com/index.php?option=com_upcomingnews&Itemid=147&headno=6
I haven't used one myself (been itchin' to).