According to ESET and Sophos, hackers have successfully ported a previous LINUX targeting malware to Mac OS X. This malware is delivered via aTrojan Horse attack over an IRC chat room. The Trojan Horse (malware disguised as something useful) attack then deposits code into Mac OS X to turn it into a robot for attacking specified web sites in what is called a Distributed Denial Of Service (DDOS) attack.
If you do not enter IRC chat rooms (you would know if you do) then you should not have to worry about encountering this Trojan horse. As always, do not download nor install applications from untrusted sources.
You can find more info at MacWorld.com, Sophos.com, ESET.com and others.