Reproduced with Inkscape 0.48.4 and current trunk r13371 on OS X 10.7.5.
> The result is hidden.
Not necessarily hidden: AFAICT this depends on the matrix: if the group is only rotated, the clipped object is still visible, but clipped incorrectly. In this case it seems that part of the transform is applied incorrectly to the clip-path (or mask) itself, as can be noticed when releasing the clip instead of undoing it (if parts of the clipped object are still visible).
Reproduced with Inkscape 0.48.4 and current trunk r13371 on OS X 10.7.5.
> The result is hidden.
Not necessarily hidden: AFAICT this depends on the matrix: if the group is only rotated, the clipped object is still visible, but clipped incorrectly. In this case it seems that part of the transform is applied incorrectly to the clip-path (or mask) itself, as can be noticed when releasing the clip instead of undoing it (if parts of the clipped object are still visible).