(Denny Crane: "I Don't Want To Socialize With A Pinko Liberal Democrat Commie. Say What You Like About Republicans. We Stick To Our Convictions. Even When We Know We're Dead Wrong.")
Yes i never seen a virus on a unix or linux box, but i wouldnt be doubting someone out there is trying to exploit one of the thousands of security vulnerabilities on MAC OSX.... OSX might be based on BSD, but soon as steve jobs adds all those pretty bells and whistles to the operating system and complicates and bloats the code, there is bound to be some serious security flaws.
This isn't completely true. Any user can install a program in, say, his home directory. You only have to have 'root' privs if you are installing to a directory that users don't have permission to write to.
Any program installed in a user's directory will execute with the same priviledges as a user though, so while it can harm any user data writable by the user, it can't touch system files (i.e., config files in /etc)