In blender cycles camera clipping works as picture shows:
Is it bug or feature? I really need clipping with flat plane.
The clipping is a distance relative to the camera. If you take all the points at a set distance from a center, I think you get a sphere. That’s what Mathematics say. 
What you get is neither a feature nor a bug. It’s just working the way it should.
If you want another result, you’ll need another way. Altho you didn’t say what you expected, I suppose that you want an image surrounded by a black frame. Just build the black frame and attach it in front of the camera. That’s what I would do… even if I don’t understand why I would. 
It is not mathematics. It is behaviour of cycles, possible wrong.
Because in LuxRender/MaxWell camera clipping means “clipping plane”, NOT “clipping sphere”.
ps:
http://projects.blender.org/tracker/?func=detail&atid=498&aid=32255&group_id=9
http://projects.blender.org/tracker/?func=detail&atid=498&aid=34197&group_id=9
Alright, maybe it’s a bug… I must say I don’t think I’ve ever used the clipping. Most of the time I lowered it to zero. Does that count? 
Clipping can be useful when you are doing interior renderings. You can place your camera outside of the room and set clipping plane so it will cut unnecessary walls/objects from rendering.