And it seems to be most prevalent in anime. I don't think I've ever seen this in normal film (non-animated).
I just checked the ef - Melodies OP, which is full of slow-moving horizontal and vertical panning, and all of it is perfectly smooth. The video is 23.976 FPS H264 decoded using ffdshow. This video I encoded is also perfectly smooth in slow pans, although it's 24 FPS instead of 23.976. The 23.976 number actually comes from 24000/1001, which is the standard frame rate for progressive NTSC film.
http://www.mplayerhq.hu/DOCS/HTML/en...-telecine.html
That shaking might actually be caused by the encoding process, where the encoder adjusted the frame rate by simply discarding frames from the original.