Any digital camera "sees" IR as well as visible light. Take out the IR filter (that pretty much all cameras have) and they can be used as a night vision device. Simple as that.
That is how those toy NVG's work.
When the filter is removed, the camera basically has a full spectrum sensor. Add a visible light filter and you have a device that only records IR.
They do not amplify ambient light, so in low light conditions, IR light has to be added. So they will only give you as much field of vision, as you can cover with your IR light source.
But it should be said, that within that area, conventional NVD's will have a very hard time keeping up with them in terms of image quality, as the camera version can do HD resolution and above.
That said, they are close to useless. I have a system like that for IR photography, but that only works because of the nature of it's usage.
For tactical applications I would not go anywhere near those things. Not only do they have very limited range, the IR light source that you will need, will also light you up like a christmas tree. Not to the naked eye, but anyone else using NVD's will see you from a mile away.
These guys do some fantastic IR conversions: http://www.lifepixel.com/
Their stuff is top notch. In fact, I will be sending in another DSLR soon (so I can have more than one filter option).
They do UV conversions as well, which I'm also tempted to try.
Gear is a poor substitute for knowledge. And much heavier.