International copyright is a tricky area. There is the Berne Convention which set up the guidelines for international copyright protection and enforcement but not all countries are signatories to it. So if the copyright infringer is located in a country that has not signed onto Berne there is little enforcemetn that can be done. Kind of like that kid that let out the computer virus in the Phillipines who was not prosecuted because there was no law against it there.

Looking at the US Copyright office webpage they say that just about anything is copyrightable and I guess the creation of a tartan would fit into this category, like a work of art. There is also trademark which is something like the nike symbol and I believe the Burberries tartan would fit into this category, something that is intimately connected to the company where knocking it off would cause a deception, and this is more of a consumer protection/brand protection issue.