The wonderful free 3D modelling and animation program Blender becomes a whole lot more interesting when its virtual world is combined with the real world. If the real world video camera was moving then motion tracking is needed to determine how the video camera moved so that the virtual camera can be synchronised with that. This 40 minute tutorial should help get you started. Yes, 40 minutes - motion tracking isn't that easy and it takes quite a while to track even just a few seconds of video. Be aware that Blender 2.61 isn't very stable when motion tracking (at least not for me). So save often, especially before solving the camera motion or rendering the finished animation.
This video is also at
See also the excellent hour-long video by Sebastian König:
and this inspirational video:

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