Probably Marco can answer in detail about the sky, but I think that's not a bug issue.

Seams are artifacts of few 3D cards, due to rendering inaccuracies. They are invisible to the user. Unless you have a nonzero sky color or some other sky behind your cube, or have not switched DX to retail mode. So, take the seams as a feature, not a bug. They indicate that you shold disable whatever you're rendering behind your sky cube - it will not only get you seams but also lower your frame rate.

This info was added to the manual.