Not read through all the comments, however some points:
1) The aesthetics (looks) of the case cannot be patented, they can be put as intellectual property, and sometimes parts of it (e.g. logos) can be put under copyright, but the actual look is not part of patenting.
2) The functional bits can be patented, but cannot be copyrighted. You might be able to put them under intellectual property, not sure how the law would handle that one.
Patents last for only a certain amount of time. They cover any functional breakthrough, if you have some totally new mounting bracket for the PSU using gallium-dichromate nanotube lattices, then you can patent that, it's a new functional technology that has gone into the product. Your logo, you can put under copyright, it's part of the brand identity. Stuff you come up with, can sometimes be put under intellectual property, though I'm not as familiar with its limitations.
The layout of a PC case though, where does that fall under? Well, it's not a logo or some piece of design, sure you do see the layout, but it's functional, not really decoration as such, so that rules out copyright. I also suspect you couldn't make it fall under a patent, it's not a new technology, it's just a new superficial application of the technology, also, patents aren't automatically applied, so if you haven't patented something before someone copies you, then it's too late. So really, I'm not sure you can sue on grounds of functional aspects being copied, looks however are a whole other ballgame. You can theoretically go to court over copying of looks, however all you need to do to avoid prosecution is to adjust small aspects, and that can be achieved by copying the overall style from a photograph, rather than using a specification to copy each technical detail...
Conclusion: Unless copying of the specification has been done, or a patent which has been approved has been copied (post-approval of patent), then I do not believe there is anything illegal going on...