I need explanation for capturing HD video with DSLR?

I wanna buy and dslr with HD video, I can't choose between canon 550eos and nikon d5000 or nikon d3100.
I don't know what is happening with the autofocus when you capture a video, is autofocus work or I have to focus mannualy. Any explanation and tips please.

I believe the D3100 is the only one that autofocuses when shooting a video. The others will autofocus just before starting the video, but requires that you manually focus during the video if you need to change focus. You will need to see video samples from each camera to see how bad the jello effect is.

Remember, the primary reason for getting the camera is for still shots. The video function is a toy on still cameras as compared to a dedicated video camera.