The real question to ask is not "Has there been?" but "Will there be?"
Given that Mac OSX is on the rise on the popularity scale I am almost dead positive we will see a lot of increase in malware for OSX, even remote attacks where the user will be infected even if he acts sensibly.
The risk of that happening to you though is fairly low. The risk of clicking something and then getting malware through that is way higher, from a risk point of view.
But of course going forward the vast majority of Mac users are going to be getting their software through the Mac App Store, and not downloading random programs off the internet. That plus the default GateKeeper config (that prevents unsigned code from being installed) will dramatically reduce the risk of the average user getting hit by trojans on the Mac platform.
Given that Mac OSX is on the rise on the popularity scale I am almost dead positive we will see a lot of increase in malware for OSX, even remote attacks where the user will be infected even if he acts sensibly.
The risk of that happening to you though is fairly low. The risk of clicking something and then getting malware through that is way higher, from a risk point of view.