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.
When that happens, Apple will devote its considerable resources to making the offender wish he was dead (he had to give up his information to get into the Mac Dev Program, remember?), and then it won't happen again.
Yes, but if someone makes an OS X malware app, they would still need to think of the distribution channel. Few will download from my-amazing-toolbar dot com. But if your malware is in official app store ... there we go.
What's to stop something like this from happening: http://venturebeat.com/2010/07/28/android-wallpaper-app-that...
Why you would wait until there is an app store that gives a large company a lot more information on who you are and where the authorities should come find you before inserting malware, I can't even begin to guess.