It's a very wide spread misconception that Macs are immune to viruses (or virii if you're 1337). This is NOT (nor has it ever been) true. There are far fewer viruses that can infect Mac, yes... but they are still out there. The main issue is that the complacency of the end users make them the easier target.
The bigger immediate threat at the moment is a new backdoor trojan that has been detected out in the wild. Named Tsunami, it allows a cracker remote access to your Mac and make it do horrible things (Such as download porn!)
Sophos, one of the leading Apple Anti-virus software providers has detected it and has issued an update for their anti-virus software package. For Mac users that haven't downloaded an anti-virus yet for their Macs, you can get Sophos' free home version here.