I've just been reading more about the Mac Trojan, and it seems there are now 32 variants of it. This evolution shows the serious nature of the attack. Security experts are saying the purpose of the attack is to direct people to porn web sites which pay for the traffic. This means people are making money, which also means we can expect to see more attacks like this in the future.
The article "Multiplying Mac Trojan not epidemic yet" on CNET nicely outlines the significance of this, while pointing out that we don't have to overreact either. When you combine this with a recent announcement of a security vulnerability in Symantec AntiVirus for Mac, you start to realize that Macs might not be as impervious as some people think.