It's not illegal assuming you are using a purchased copy of leopard, but it is against Apple's EULA. Violating a EULA is NOT illegal (not in the US, at least not YET.
) A EULA is a form of contract. If you violate a contract, you are not breaking the law, you are breaking your contract. I personally don't give a crap what Apple's EULA says, but this board, or at least the people who run it do, so discussion here wouldn't be acceptable.
Only thing that could possibly be illegal would be making and distributing the actual patches, as they contain code that Apple would claim is their property and copywrited, and therefore would be illegal to distribute in a lot of countries. That's it. The act of installing it, or, you modifying it yourself to run on a PC is in no way illegal, even if corporations would like to make you think it was.
If I payed for it, I own it. I can open it and make any modifications I like to it, and then use it for whatever I see fit.
Anyway, there are sites dedicated to the subject, so it's not even necessary.