There's this news about 600K+ Mac been infected with "Flashback trojan" (some reports: here, here and here), make sure you patch your Mac ASAP following this instruction from Apple!
According to the instruction, it's "Java 1.6.0_29" that's having lots of problems and should upgrade to "1.6.0_31". And to check what version of Java runtime you have installed in your Mac, you have to run "java -version".
I tried it on my MacBook and this is what I got:
MacBook-Pro:$ java -version
No Java runtime present, requesting install.
Ha ha, I didn't even have it installed...So looks like my Mac is safe from this specific trojan?
Just learned that it's because my MacBook comes with Lion and Lion doesn't install Java by default. But I found Safari still have "Enable Java" ticked by default, and have to go to "Preference -> Security" to disable it. But if you are running Snow Leopard that would be a different story.