I've seen plate shots being made while on film sets, but I'm not sure exactly how they're defined.

My impression is that you can have action or motion in the frame, but there will be computer generated images added in post production.

Does the camera have to be static, or "locked-off"? I'm just curious.

You can add CG in the scene like the movies, but you have to film on a set with a blue or green screen to use chroma key and remove the background. Or you can add the CG without chroma key but it will be a lot more difficult, you will have to edit it frame by frame.
The camera doesnt need to be static, there is a lot of programs called "Camera Tracker" that can identify the camera movement when you are compositing. One of the best is Boujou.

Thanks for the information. I've definitely seen the bluescreen and greenscreen shots before.

I think the last plate shot I saw not using that technique was on Hairspray, which had a very considerable budget. I'm looking forward to seeing the results in the film this summer.

Cool, this sounds interesting. I did a movie like this, without the CG some five years back, it was short and cool, but nothing fancy. Maybe I must call up my buddy again and see if we can do one more, with CG init