The buzz is that virus creators have found a way to pick on Mac users (Washington Post article). As I understand the approach, Mac users must be tricked into downloading a file purported to be a picture but that is actually a program. When opened, the program attempts to send itself to anyone on the users iChat list.
The logic works like this – here is a secret file, open it, run it, and see what happens (I am being facetious here). The danger would seem to be in situations in which you receive such a file from someone you know and do not stop to wonder why an image file would come in a compressed form, etc.