I may be missing something but it doesn't look like this thread has answered the initial question (although the discussion about the theoretical possibility of macs getting a virus is interesting). The reason I'm posting is because I have seen the same weird symptom on my Mac Mini recently. I have been using it for nearly a year now with nothing odd happening. However I have recently noticed that when the screen saver is on (which I have set up to use my iPhoto library) sometimes the pictures are obscurred by weird looking cartoon graphics/icons. Two that come to mind are of a sunflower and of a purple monster face. They are quite blocky looking and despite the debate in this thread my first thought was virus. You'll have to trust me that I am not making this up, the images in iPhoto do not have these weird additions and I have not been experimenting with hallucinogenic drugs. I'm not saying this IS a virus, just asking the question what people think it is and how I can get rid of it.
These little fellas?


If so then you might want to remove your iChat icons from your iPhoto library.