It's a fact that Macs are much safer to use--they are not 100% hack-proof, but they are a lot more safe than PCs. However, PCs are much more versatile. I am not anti-PC, I'm pro-PC and pro-Mac. Each are good for what they excel at.
There are two ways I've heard of the new Mac OS being hacked, and both ways involve the owner of the Mac being completely at fault.
1) Turn your Mac into an Apache server. This basically gives you all the advantages -- and security problems -- as any other Apache server. If you don't know your way around specific Apache security issues, you should never do this. I would advise not doing this anyway, but at least know the risks before pushing buttons.
2) Download warez. Basically, you download illegal software, you tend to do business with pirates. I know there was a confirmed case of a man downloading a bootlegged copy of Microsoft Office and he ended up completely hacked.
I've yet to hear of someone simply breaking through an out-of-the-box Mac as a matter of security loopholes in the OS. I'm sure they're there, but Norton hasn't warned me of any needed fixes or patches (and they've been guilty of warning Mac users at every whisper of a rumor, almost always false).