[Numberzz]

Yes, it's illegal. Do I think Apple will ever catch you? No. The only really plausible way that anyone could ever catch you is if someone caught you torrenting one of the modded releases that matched your hardware.
Now I'm staff on the OSx86 forum (insanelymac.com), and no one has ever said that they've gotten an email from Apple, or been sued or anything like that. If you want to do it? Fine, it'll be hard, but after you get the vanilla kernel up and running, you can even run software updates right from the Software Update application. It's really a lot of fun, but getting it to run for the first time can be time consuming.