first they'd need easy access to your box. By this I mean a non-firewalled connection, and an opening to exploit. Most of the 'hackers' out there are relatively unskilled and use scripted toolkits to perform their 'hacking' actions. Since that's the case and there are no known os x exploits in the wild at this point (other than some trojan horses) I'd say you're pretty secure. We portscanned my mbp one day at work just to see if there were any obvious security holes, there weren't. Mitnick be darned, I don't consider social engineering to be actual "hacking"
So what do I think they'd need? First and foremost, something that is generally lacking in the scriptkiddie arena, skill. Second they'd need some actual will to hack your mac, not sure what kind of information you're storing on it, but it's probably unlikely to be worth the amount of time and effort likely required to breach it (at least in most cases). Security by obscurity? No, just that anything that's connected to a public network can be compromised, some are just harder than others.. and generic thieves tend to find the easiest or.. 'soft' targets.
