I'm seeing a serious bug on the latest ATI cards when using the mod instruction in cg.
int worldX = ((int)worldPosition.x)%(width);
This seems like a major flaw - works fine on my old FireGL VX and also on nVidia cards but it's very broken on my new HD4850. It also had problems on my laptop's HD2400 but newer drivers seem to fix it.
I can provide a screenshot of the result.