I have the same problem which I think is related to the ATI Graphics Card I have, I cannot see any thing else it could be, then again I might be wrong and it could be some thing else after all this software is still a work in progress.
Its like the sky is all broken up in the 3D Preview instead of been smooth, I am just wondering weather if a graphics card is bellow a certain threshold for on board memory weather that has implication's for the way the Open GL Preview will display, as when you render the scene every thing is how it should be.
It could just be some thing to do with the way that TG2 handles Open GL some thing to look into.
Regards to you.
Cyber-Angel