More than 600,000 Macs have reportedly been hit by a trojan called BackDoor.Flashback that exploits a Java vulnerability in Mac OS X to take control of machines and add them to a botnet. There may be more than 4 million compromised webpages currently running the exploit.
A Java update released by Apple on April 3rd fixes the vulnerability and prevents infection, but won't remove the malware from an infected machine. F-Secure has posted detailed instructions for manually detecting and deleting Flashback.
Note that Mac OS X 10.7 doesn't come with Java preinstalled, and is thus not vulnerable to this attack unless you've installed Java yourself. Users of Mac OS 10.6 and below should definitely be on the lookout for this Trojan, though.