That's a rather challenging move, since you're talking about an orbit-style move rather than a rotation, if I understand it right.
Ah yes - I suppose I am!!! I hadn't really thought of it like that but I guess it's not a rotation in the strictest sense of the term... Is that type of move possible (cleanly) in PE - or would that be more in line with PPro/After Effects? I can kind of get the effect I'm looking for by adjusting the anchor point at the final key frame but, to be honest, it looks a little bit amateurish as it's difficult to do without also affecting the poisitioning of the object in question.
For orbital animation, Adobe After Effects would be the best program to use. PrPro will have the same limitations as PrE in that respect.
Good luck and happy editing,
The reason the clip moves is that because it keeps the "old" position parameters, you need to adjust them accordingly.
Also, have you tried Effects-Transform? It lets you animate and keyframe the Skew parameter of the clip which may improve your effect. For instance, if you have a 720p project, enter both the Anchor Point and the Position as 1280, 720. Then animate the Rotation and Skew as you want.