Well, here's a thought- if you pry off the optics and everything, and get down to the bare detector, more likely than not you'll see a little piece of material stuck to it. Pop that off, and your camera will suddenly see infrared like crazy.
If you REPLACE that little bit of material with, say, a piece of exposed 35mm film, you'll filter out visible light. You now have a super-cheap infrared-only camera.
Note that this is NEAR infrared- you still need a light source for it to work. But infrared LEDs are a dime a dozen (even Ax-man sells them).
What you do with it then is up to you.
Unfortunately, the process for taking analog video signals (your cams probably push out NTSC) and converting it to digital is so painful, 99 times out of 100 it's simpler just to buy a camera that dumps out digital data into USB or FireWire.
And, FWIW, using an Arduino to do this would be an epic challenge. The Arduino can't possibly keep up with the data rate, but it COULD (possibly) arbitrate the data flow through a CPLD into some type of a PC-bus-friendly linkage (FireWire, USB, etc). We have several products that use an a processor very similar to the Arduino's to do just that. It would NOT be easy.