Did you recently upgrade to Snow Leopard? If so, your computer has a security vulnerability that you may not be aware of. An antivirus company named Sophos has found that Snow Leopard actually installs a security downgrade (via AppleInsider) from the most up-to-date version of Flash. This means that, even if you're diligent about keeping your security updates current on your machine, you could be at risk for attack.
This change, which installs version 10.0.23.1 of Flash instead of 10.0.32.18, is made without warning the user. Considering how ubiquitous Flash is, and that it is a major target of attacks by hackers, this represents a substantial lapse in judgement for Apple. Snow Leopard does offer limited malware protection, but (right now) only against two different Trojans.
It's unfortunate, but Macs have developed a reputation for being somehow 'immune' to viruses, malware, and other unpleasant acts of hacker. In reality, Macs are still vulnerable to attack. It isn't nearly as prevalent for them as it is for PCs, but that is changing as Apple computers become more and more popular.
Information has just come out that Apple is already testing update 10.6.1 for OS X. It's unknown if this patch, which will contain some security updates, will fix this issue. It will probably be a week or two before we get any concrete information about the contents of this update.