i guess it's safe to say that if you have to debate whether something is illegal or not, it's more than likely illegal.
however it just makes me a little upset that if i personally built a pieced together, part by part, mac pro in my own tower, with the same parts as the mac pro, it would be illegal to use it with OS X
oh well,
so much for DIY
If you built it with the same parts as a Mac Pro I'd bet it costs quite a bit more than a Mac Pro since you'd have to by the MB from Apple (I'm guessing their not cheap) along with the case and other bits that Apple is the exclusive vendor for. You could get the HD, RAM, and Video card from 3rd parties, and while you'd save a bit on the RAM, my guess would be that the premiums paid to get the MB along with forgoing the discounts Apple would not be offset by this savings which you can also achieve by buying with the minimum amount of RAM and buying 3rd party for expansion.
Not to mention that it would probably be legal at that point since the case MB and some other bits would be "Apple Labeled."