It's not so much that it's easier to Hack mac OS X than Mac OS 9, it's more like ... it's possible. Mac OS 9 was not based on IP, and you had to jump through hoops to get IP working. There wasn't much of a chance you'd "Accidentally" leave something open. It'd be like accidentally chopping down trees and building your own house.
Mac OS X has a lot of native IP stuff, and as such, there are a lot of possibilities about how communication usually contained on your computers might be intercepted by or replaced by communication over the internet. But (and it's a big but) Mac OS X has done a LOT of things right in terms of security. It's based on BSD, which is rock hard, and they dominantly kept the default off philosophy of security.
While Windows has ... essentially gone out of its way to enable bad things to happen on Soooo many levels. I could totally go off here, but I'll refrain. Windows is about as secure as a sponge is water tight.
In terms of difficulty to hack, from 0 through 10, 10 being really hard to hack (like steal information or take control) then Mac OS 9 was a 10, Mac X is a 9, Win NT and Win2k are a 5 or so, win XP I'm estimating about a 4 so far, Win 95 and 98, like a 3. RedHat fluxuates from release to release, I'd go from 5 to 9 there.
Now there's a different type of hack, not breaking into, just breaking, like crashing. I'm surfing the internet and then suddenly, I'm not. For that, Mac 9 is the easiest, followed shortly after by Win 95 and 98, XP next, then NT and 2k, RedHat is right there with NT and 2k. Then there's Mac OS X, and then real BSD. BSD is essentially proven secure and solid. I'm a big fan.
If you are afraid of being hacked, you can get any sort of firwall up, but mostly, just don't enable file sharing or remote access. I've had my stuff looked at by security firms, and my Mac X boxes have been bulletproof. If you want to be afraid of hackers, run Windows.