The Mac OS isn't immune to user stupidity:
The iServices.A Trojan horse is an attack being distributed via BitTorrent, where it's disguised as a bootleg copy of the new iWork 09. Once installed, the malware takes administrator access and connects to remote servers over the Internet, where it can be given additional instructions as the author commands, from installing additional malware to stealing information off the Mac in question. The malware creator can also take complete remote control of any compromised machine.
The takeaway lesson - pay for your software!