As MacOS isn't as popular as windows, you won't get viruses as easily, but they do exist, and there's no reason why someone shouldn't send out an email with a mac virus. Or an amiga virus for that matter.
I'm thinking of geting a Mac sometime to replace my intel pcs. (I already have two mac laptops).
Also, I don't use any security crap at all. Yes, I password protect my user account, and my wireless network is protected with the password 'hello' (there's a tip for those of you who go past my house).
I really don't like security software. It moans about not being updated every 5 minutes. It takes up loads of system resources meaning I can't do alot else with the computer while it's running, or at least I can't when I'm using windows xp as that takes up loads of resources. It increased boot times. My amiga 1200 with a blizard 1230/50MHz with WB3.1 boots up in 6 second. Windows 2000 with security software takes ages.
Also, you may end up with dificulty connecting to other computers/sharing stuff from your own computer/using your computer as a web server/ p2p/whatever because it might block stuff.