Spheremaps can't be dynamic, I guess, unless you can find a way to render those "180 degrees pictures", and map it right, so it looks like a proper reflection. The nice thing with cubemaps though, is that I can make the game pre-render them in the loading section.
Even panoramic images may not look correct, due to polar distortion.
It would be ideal (and probably such a 'perfect-world') if to have true reflective capability as a texture flag! Or if Mirrors weren't infinite and flat all the time!