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.
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.
What's to stop something like this from happening: http://venturebeat.com/2010/07/28/android-wallpaper-app-that...
In the days when I used Windows, I naturally only downloaded "official" apps and avoided any sort of non-famous/non-open source shareware for years -- the spyware/toolbars/etc. are just too rampant. I have an Android phone that I use for development, and there, I too am incredibly cautious about what I download. The "open" Android platform actually prevents me from discovering potentially amazing indie software because, well, I just can't trust it.
Revoke the developer's code signing keys Freeze the developer's payments Alert the authorities about the developer Revoke the app from the app store Push an "upgrade" notification to every registered user to remove the application Or even more drastic, engage the application kill switch
I'm really not sure how you think a fully managed software platform will have more malware problems than the run-arbitrary-code-off-the-internet system we have now. It seems like it's actually a killer solution to the problem.
What's to prevent people from inserting malware into their applications? Unlike the iPhone, OSX isn't sandboxed. And there's no way Apple's approval process can possibly check for possible badness hiding in the compiled code.
Unless, of course, they are building a special sandbox for these OSX apps as well. Which means that you'll have to write apps specifically for this platform.