Right now I just don't download random OSX apps for fun. I only download applications that I really need - and I make sure it seems like a reputable company. And I certainly wouldn't download fart apps, or a lot of the other 5-second novelty apps that have a lot of traffic on the app store.

The first time an OSX app store has malware in it, it's going to be big news. And it's going to make Apple look bad, since by putting it in their store their effectively condoning it.