Actually, I use my own XCode project to workaround this problem.
It is a modification from the GLBasic's XCode autogenerated project, to make my project "Universal" cappable. I did it long time ago, from XCode 3.X early days

If I compile using my own XCode project, I don't have rounded corners or black lines at bottom (tested under iPad 2 and iPad 3)...
I was trying to found the difference between my project and the GLBasic's generated project without luck

I can't see any substantial difference related to the rounded corners...
I was testing generating a fresh new GLBasic's project, changing all its XCode parameters without results.
I only can make dissapears the corners and the black line by copying my own (and old) .xcodeproject folder and its info.plist to the new project's folder (the two are renamed, to not have to overwrite anything and avoid problems each time that GLBasic compiles, because I know that some time ago it could overwrite the XCode changes).
Hope this helps to found the bug. Regards.