Well, first of all, IANAL. This is actually a rather murky legal issue, so if you're really worried about your legal exposure, consult a bona fide legal professional in your local country. Also, I'm going to address this as it relates to US law, because if I try to make it generic to all world laws, well, that would be hard.
That being said, you are really facing two issues. The first is copyright infringement, which is a criminal offense that can carry serous penalties, though the enforcement is almost impossible in the case of an individual. Luckily, installing a legally purchased copy of Mac OS X on your Hackintosh does not require that you violate copyright, with one wrinkle: the DMCA. The Digital Millenium Copyright Act makes the process of breaking even rudimentary encryption to "copy" a copyrighted work illegal in itself. Apple claimed against Psystar that it violated the DMCA when it "illegally circumvented Apple's technological copyright-protection measures." But it's not clear that what an individual needs to do to install Mac OS on a non-Apple computer necessarily violates the DMCA. See this OSNews article from a couple of years ago on the topic. So as far as criminal offenses go, it's probably possible to make a Hackintosh, even in the US, without violating copyright, though if you're going to be a stickler, it's probably going to be a more time-consuming process.
The legal aspect that's simultaneously more clear and more cloudy is the civil aspect: the license agreement. To use Mac OS X or any software, including open source, you implicitly agree to a contract with that software's author. That's the End User License Agreement, or EULA. Apple's EULA says, in essence, that you're only allowed to install it on an Apple-brand computer. So it's clear that if you make a Hackintosh, you're violating the EULA and could be subject to to civil legal action if Apple decides to pursue it. So it's simple, right?
Not so simple. You can make a contract, and you can even get someone to agree with the terms, but that doesn't necessarily make it a valid contract. There are all sorts of factors that might make a contract invalid, and particularly when one of the parties only "agrees" in the loosest sense, such as with so-called "shrink wrap" contracts like EULAs. Even a contract signed and notarized and sealed with wax aren't necessarily valid. There are many factors that can cause a contract to be void from the get-go. (There are a lot of these factors, and each one of them is a potential rabbit hole of common law precedents and vague analyses, with several that could possibly apply to a shrink wrap EULA if it were to be vigorously challenged in the courts. There's a reason that civil court cases often go on for months or even years.)
But to get back to your original question, it's not "illegal" to violate a contract. When you decide to cancel your cable TV or mobile phone service early, you're breaking your contract. When you stay parked in a parking space longer than the sign allows you, you're violating a contract. What happens is that you may be on the hook to pay the penalties that are specified, or will at least be obliged to argue with the other party before a judge. The consequences of violating a contract are usually limited to a monetary penalty.
So, in short, it's probably possible to do what you propose without breaking any laws, but you will be running afoul of Apple's license, so you'll have to be comfortable with Apple's stern disapproval. You'll also be running the risk that they could come after you in court for violating a possibly-invalid contract. So I wouldn't go taunting Apple's legal team on YouTube while you play with your Hackintosh.
Also see this Low End Mac article for a similar take on your question.