For Lion and Mountain Lion, an upadate can be downloaded from the Oracle website.
For Snow Leopard an update is available using Software Update.
The latest version for Leopard appears to be available here
but is dated June 2011. I don't know whether any update is available through Software Update but I rather doubt it. Are you really using Leopard as your signature states? If so you should certainly turn Java off in your browsers (it's probably wisest to do this on any system anyway) and to consider upgrading to Snow Leopard or higher if your Mac can handle it.
Apple barred Java from running on Macs, leaving companies that rely on Java plug-ins out in the cold.
Apple blocked Java 7 Update 11 by adding it to the banned list in XProtect.
This was the second time in two weeks that Apple has blocked Oracle's code from running on Macs. This time Java is blocked through Apple's XProtect anti-malware feature.
Java has come under fire as the means by which hackers have been able to gain control of computers. In April 2012 more than 600,000 Macs were reported to have been infected with a Flashback Trojan horse that was being installed on people's computers with the help of Java exploits. Then in August Macs were again at risk due to a flaw in Java, this time around, there was good news for Mac users: Thanks to changes Apple has made, most of us were safe from the threat.
Unwilling to leave its customers open to potential threats Apple decided it's safer to block Java entirely.
Macs running OS X Snow Leopard and beyond are affected.
UPDATE for those running Lion or Mountain Lion:
Oracle on Friday February 1 released a new version reportedly addressing vulnerabilities seen with the last build.
Apple disabled Java 7 through the OS X XProtect anti-malware system, requiring users to have at least version "1.7.0_10-b19" installed on their Macs. The release dated February 1 carries the designation "1.7.0_13-b20," meeting Apple's requirements.
Oracle "strongly recommends" applying the CPU fixes as soon as possible, saying that the latest Critical Patch Update contains 50 new security fixes across all Jave SE products.
Update foror Snow Leopard users:
Apple have issued update 12 for Java for OS 10.6:
Note: On systems that have not already installed Java for Mac OS X 10.6 update 9 or later, this update will configure web browsers to not automatically run Java applets. Java applets may be re-enabled by clicking the region labeled "Inactive plug-in" on a web page. If no applets have been run for an extended period of time, the Java web plug-in will deactivate.